Write pdb files for PDB CCD

What

• See #3 (closed) for sdf files
• in addition we need method to write old style PDB format files.

How

• RDKit includes code to write PDB files
• Note that it is important the PDB files produce are well formed with correct atom names and residue names.
• In addition the occupancy and temperature factors should be well formed.
• probably also neccessary to include dummy CRYST1 card
• Testing should include comparison to existing files at PDBeChem as well as loading in coot, pymol, litemol.