relax SVN archive

Modified the __get_chemical_name() to handle unidentifiable residue names.

Missing chain_ids are now caught and replaced by empty strings.

HETATM records with no residue info are now skipped in the HET record construction.

Fix for the main write_pdb() method.

Now the file object is sent into the specific write_pdb() method rather than the file path string.

Modified how non-standard residue information is collected and stored for future writing.

Fixes for the write_pdb() method for handling multiple structures better.

Updated the __validate_data_arrays() method to handle multiple structures.

The structure_container instance is now passed into the method.

Converted the write_pdb() function to loop over all structures.

The struct_index is now passed into the write_pdb() method.

Added the force arg to the structure.write_pdb() user function.