Convenience and atoms.
Convenience and atoms.

1. Added several convenience methods for creating matt.PDBArg values
from PDB entires and/or chains.

2. Implemented PDB 'ATOM' record parsing and added a corresponding
'Atom' type. Currently, it focuses on only the carbon-alpha ATOM
records, but also slurps up the others too.

TuftsBCB/fragbag