Piecewise syntax is changed to allow for a more sensible definition of
the otherwise condition. Before, an expr/cond pair was specified where
cond was given as True. Now, otherwise can either be passed as the final
argument after a sequence of expr/cond 2-tuples. The old otherwise
syntax using (otherwise, True) raises a deprecation warning.

The ExprCondPair class is removed in favor of Tuples to store the
expr/cond pairs.

The '.as_leading_term()' function is changed so that it takes the sum of
all the expressions and, if it is not NaN, the otherwise statement, and
returns the leading term of the sum. Ideally, '.as_leading_term()' would
return a Piecewise function, but doing this breaks the evaluation of
'abs(x-a).nseries(x, 1)'.

Piecewise no longer accepts an Interval as a condition. To implement the
same functionality, either the '.as_relational()' or '.contains()'
methods of Interval can be used. Piecewise also no longer accepts
numbers as conditions.

The properties '.exprcondpairs' and '.otherwise' are implemented to
easily access the expr/cond pairs and otherwise statement, respectively.

Addresses issues: 2101, 2567, 2626, 2710, 2726

Changes allowing evaluate keyword to override default behavior of
clearing nested Piecewise expressions. More testing of new Piecewise
evaluate keyword behavior, including an XFAIL test related to Issue
3025, where evaluate=False does not override behavior when bools are in
the conditions.

Includes new tests and doctests for '.exprcondpairs' and '.otherwise'
properties.

Re-enables testing for expressions in expr/cond pairs and the otherwise
statement to be Expr or GeometryEntity. GeometryEntity is needed due to
the use of Piecewise in the geometry module for '.arbitrary_point()'.

Piecewise evaluation now returns the first expression which is
explicitly True. If no conditions are True, the otherwise expression is
returned. Evaluation can be triggered by setting the 'evaluate' keyword
to True. When a boolean is given as a condition, evaluation is enabled
unless specifically disabled by setting 'evaluate' to False. In this
case, any explicitly False conditions are removed and any explicitly
True conditions are given as the otherwise statement. This is done so
bools are kept out of args, see Issue 3025. The collapsing of nested
Piecewise functions is moved from eval to the __new__ method. This is
performed by default, but setting 'evaluate' to False will disable it.

Added a .doit() method to Piecewise functions. This allows for
forced evaluation of the Piecewise function. If you want to call doit()
to evaluate args but not evaluate the Piecewise function itself,
evaluate=False can be added to the hints.

Change '.doit()' keyword for evaluating Piecewise objects from
'evaluate' to 'piecewise' and set other functions to use this to
avoid evaluation of Piecewise when required. Having no expr/cond pairs
is treated like having bools in the conditions.

Improve documentation of Piecewise objects, specifying how the new
evaluation functionality works, how it is overridden and the limitations
posed by not being able to have bools in the args.

Change behavior of Piecewise so if bools are given as args and it is
given the keyword evaluate=False, it raises a NotImplementedError. This
can be changed once classes implementing boolean logic that subclass
Basic are implemented. Changes are made in meijerint to parse the args
of a Piecewise function used in that module which may have bool
conditions, but should not be evaluated. This can be changed to use
evaluate=False when the Basic boolean classes are implemented.

Change checking in Piecewise to take expr/cond pairs only in tuples,
Tuples or lists, all other expressions go to otherwise. Drop the type
checking for the exprs and otherwise statement.

Conflicts:
	sympy/functions/elementary/piecewise.py
	sympy/printing/tests/test_str.py

Conflicts:
	sympy/solvers/tests/test_ode.py

Conflicts:
	sympy/printing/tests/test_str.py
	sympy/stats/crv_types.py