Add PDBx/mmCIF support.

[primaryodors]

RCSB does not support downloading of PDB files for strands over a certain threshold length.

• The Protein class should be enhanced to allow loading of .cif files;
• mmCIF supports 3 letter chain IDs, which will affect the LOAD command in Pepteditor;
• A new download source will have to be created, something like RCSB_mmCIF.