Rewrote the docstring of the 'self.write_pdb_file()' function to include PDB record descriptions.
Desriptions of the PDB records HET, HETNAM, FORMUL, HETATM, TER, CONECT, MASTER, and END including
tables of columns, data type, field, and definition have been added. This info comes from the
"Protein Data Bank Contents Guide: Atomic Coordinate Entry Format Description" version 2.1 (draft),
October 25, 1996.
