Python interface compatibility updates

The Python interface, as it stands, is not compatible with Python3 and with the recent versions of the Numpy API.

I've committed, for the moment in the branch py3fixes, a small set of improvements:

numpy now gets detected correctly in both Python2 and Python3 environments (932e1a00);
calls to the deprecated PyArray_FromDimsAndData() have been replaces with calls to PyArray_SimpleNewFromData() (932e1a00)
other small compatibility fixes

If anyone can test and report, it will be greatly appreciated.

Cheers, Andrea

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