Implement some comparisons and arithmetics for GncNumeric

Derive some numeric comparisons and arithmetics from Fraction
class
https://github.com/python/cpython/blob/3.7/Lib/fractions.py
and general information from
https://docs.python.org/3/library/numbers.html#numbers.Integral

These methods are bound closer to the gnucash-C-api than those
from the fraction class.

This is not the full set of comparisons and arithmetics, needs
to be extended. It would be good to extend tests to cover this.

bindings/python/gnucash_core.py

@@ -28,6 +28,8 @@
 #  @author Jeff Green,   ParIT Worker Co-operative <jeff@parit.ca>
 #  @ingroup python_bindings
 
+import operator
+
 from enum import IntEnum
 from urllib.parse import urlparse
 
@@ -426,16 +428,18 @@ def __args_to_instance(args):
         elif len(args) == 1:
             arg = args[0]
             if isinstance(arg, int):
-                return gnc_numeric_create(arg ,1)
+                return gnc_numeric_create(arg, 1)
             elif isinstance(arg, float):
                 return double_to_gnc_numeric(arg, GNC_DENOM_AUTO, GNC_HOW_DENOM_FIXED | GNC_HOW_RND_NEVER)
             elif isinstance(arg, str):
                 instance = gnc_numeric_zero()
                 if not string_to_gnc_numeric(arg, instance):
                     raise TypeError('Failed to convert to GncNumeric: ' + str(args))
                 return instance
+            elif isinstance(arg, GncNumeric):
+                return arg.instance
             else:
-                raise TypeError('Only single int/float/str allowed: ' + str(args))
+                raise TypeError('Only single int/float/str/GncNumeric allowed: ' + str(args))
         elif len(args) == 2:
             if isinstance(args[0], int) and isinstance(args[1], int):
                 return gnc_numeric_create(*args)
@@ -451,6 +455,133 @@ def __args_to_instance(args):
         else:
             raise TypeError('Required single int/float/str or two ints: ' + str(args))
 
+    # from https://docs.python.org/3/library/numbers.html#numbers.Integral
+    # and https://github.com/python/cpython/blob/3.7/Lib/fractions.py
+
+    def _operator_fallbacks(monomorphic_operator, fallback_operator):
+        """fallbacks are not needed except for method name,
+        keep for possible later use"""
+        def forward(a, b):
+            if isinstance(b, GncNumeric):
+                return monomorphic_operator(a, b)
+            if isinstance(b, (int, float)):
+                return monomorphic_operator(a, GncNumeric(b))
+            else:
+                return NotImplemented
+        forward.__name__ = '__' + fallback_operator.__name__ + '__'
+        forward.__doc__ = monomorphic_operator.__doc__
+
+        def reverse(b, a):
+            if isinstance(a, (GncNumeric, int, float)):
+                return forward(b, a)
+            else:
+                return NotImplemented
+        reverse.__name__ = '__r' + fallback_operator.__name__ + '__'
+        reverse.__doc__ = monomorphic_operator.__doc__
+
+        return forward, reverse
+
+    def _add(a, b):
+        return a.add(b, GNC_DENOM_AUTO, GNC_HOW_RND_ROUND)
+
+    def _sub(a, b):
+        return a.sub(b, GNC_DENOM_AUTO, GNC_HOW_RND_ROUND)
+
+    def _mul(a, b):
+        return a.mul(b, GNC_DENOM_AUTO, GNC_HOW_RND_ROUND)
+
+    def _div(a, b):
+        return a.div(b, GNC_DENOM_AUTO, GNC_HOW_RND_ROUND)
+
+    def _floordiv(a, b):
+        return a.div(b, 1, GNC_HOW_RND_TRUNC)
+
+    __add__, __radd__ = _operator_fallbacks(_add, operator.add)
+    __iadd__ = __add__
+    __sub__, __rsub__ = _operator_fallbacks(_sub, operator.sub)
+    __isub__ = __sub__
+    __mul__, __rmul__ = _operator_fallbacks(_mul, operator.mul)
+    __imul__ = __mul__
+    __truediv__, __rtruediv__ = _operator_fallbacks(_div, operator.truediv)
+    __itruediv__ = __truediv__
+    __floordiv__, __rfloordiv__ = _operator_fallbacks(_floordiv, operator.floordiv)
+    __ifloordiv__ = __floordiv__
+
+    # Comparisons derived from https://github.com/python/cpython/blob/3.7/Lib/fractions.py
+    def _lt(a, b):
+        return a.compare(b) == -1
+
+    def _gt(a, b):
+        return a.compare(b) == 1
+
+    def _le(a, b):
+        return a.compare(b) in (0,-1)
+
+    def _ge(a, b):
+        return a.compare(b) in (0,1)
+
+    def _eq(a, b):
+        return a.compare(b) == 0
+
+    def _richcmp(self, other, op):
+        """Helper for comparison operators, for internal use only.
+        Implement comparison between a GncNumeric instance `self`,
+        and either another GncNumeric instance, an int or a float
+        `other`.  If `other` is not an instance of that kind, return
+        NotImplemented. `op` should be one of the six standard
+        comparison operators. The comparisons are based on
+        GncNumeric.compare().
+        """
+        import math
+        if isinstance(other, GncNumeric):
+            return op(other)
+        elif isinstance(other, (int, float)):
+            return op(GncNumeric(other))
+        else:
+            return NotImplemented
+
+    def __lt__(a, b):
+        """a < b"""
+        return a._richcmp(b, a._lt)
+
+    def __gt__(a, b):
+        """a > b"""
+        return a._richcmp(b, a._gt)
+
+    def __le__(a, b):
+        """a <= b"""
+        return a._richcmp(b, a._le)
+
+    def __ge__(a, b):
+        """a >= b"""
+        return a._richcmp(b, a._ge)
+
+    def __eq__(a, b):
+        """a == b"""
+        return a._richcmp(b, a._eq)
+
+    def __bool__(a):
+        """a != 0"""
+        return bool(a.num())
+
+    def __float__(self):
+        return self.to_double()
+
+    def __int__(self):
+        return int(self.to_double())
+
+    def __pos__(a):
+        """+a"""
+        return GncNumeric(a.num(), a.denom())
+
+    def __neg__(a):
+        """-a"""
+        return a.neg()
+
+    def __abs__(a):
+        """abs(a)"""
+        return a.abs()
+
     def to_fraction(self):
         from fractions import Fraction
         return Fraction(self.num(), self.denom())
@@ -673,7 +804,7 @@ def one_arg_default_none(function):
 # used for the how argument in arithmetic functions like GncNumeric.add
 from gnucash.gnucash_core_c import \
     GNC_HOW_DENOM_EXACT, GNC_HOW_DENOM_REDUCE, GNC_HOW_DENOM_LCD, \
-    GNC_HOW_DENOM_FIXED
+    GNC_HOW_DENOM_FIXED, GNC_HOW_DENOM_SIGFIG
 
 # import account types
 from gnucash.gnucash_core_c import \