MAI: improve error message and documentation

The error about requested memory did not
explain how to select the requested memory
when instantiating the class `dd.cudd.BDD`.

dd/cudd.pyx

@@ -279,12 +279,17 @@ cdef class BDD(object):
         if memory_estimate is None:
             memory_estimate = default_memory
         if memory_estimate >= total_memory:
-            print(
+            print((
                 'Error in `dd.cudd`: '
                 'total physical memory is {t} bytes, '
-                'but requested {r} bytes').format(
+                'but requested {r} bytes. '
+                'Please pass an amount of memory to '
+                'the `BDD` constructor to avoid this error. '
+                'For example, by instantiating '
+                'the `BDD` manager as `BDD({q})`.').format(
                     t=total_memory,
-                    r=memory_estimate)
+                    r=memory_estimate,
+                    q=round(total_memory / 2)))
         assert memory_estimate < total_memory, (
             memory_estimate, total_memory)
         if initial_cache_size is None:

doc.md

@@ -612,6 +612,9 @@ heuristics for deciding *when* to invoke reordering.
 CUDD is initialized with a `memory_estimate` of 1 GB. If the machine has les
 RAM, then `cudd.BDD` will raise an error. In this case, pass a smaller initial
 memory estimate, for example `cudd.BDD(memory_estimate=0.5 * 2**30)`.
+The package [`humanize`](https://pypi.org/project/humanize/) is useful for
+reading memory sizes, for example `humanize.naturalsize(1073741824, gnu=True)`
+returns `'1.0G'`.
 
 
 ### Functions