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Apparently CHARMM PSF files (particularly those printed by VMD's psfgen) can
have an insertion code tacked on to the end of the residue number, meaning that
casting to an integer will invariably fail.  We can't just ignore the insertion
code, though, since it will differentiate from the residues that came before and
after (which will likely have the same number -- same general idea as insertion
codes from PDB files). So instead we pull the insertion code off of the residue
number and extend the Residue object to accommodate that.

This is the port of the fix from ParmEd: ParmEd/ParmEd#98

I added a test for this case.