Avoid truncating 64 bit return values on 32 bit platforms
Drop bogus assertion

Comparing to sys.maxint makes little sense as all it is doing is
testing the size of python's integers. The following assertion tests
that we are getting the correct return value anyway.
Numeric return results are now long rather than int
Merge pull request #1945 from tomhughes/python

Fix numeric truncation issues in python bindings