XPLOR-NIH 2.27 updates

The following Python modules have been updated. Please download and copy the files to the python subdirectory of the Xplor-NIH distribution.

psfGen.py
seqToPSF: added ntermPatch, ctermPatch arguments to customize protein termination.
simulationTools.py
StructureLoop.writeStructure: fixed exception type encountered if pwd.getpwuid fails.