Fix PSF parser in cases where single residues are differentiated by their system
(which means that consecutive residues may have the same residue number _and_
name, but are different residues).
Merge pull request #483 from swails/fix_charmm_psf

Fix residue assignment in PSF parser
XmlSerializer generates XML directly rather than going through TinyXml
Merge pull request #484 from peastman/xml

XmlSerializer generates XML directly rather than going through TinyXml