Completion of generic_fns.structure.main.load_spins() and allowed atom selections in atom_loop().
The generic_fns.structure.main.load_spins() function now uses the structural data object API method
atom_loop() to generate the molecule, residue, and spin sequence in the relax data store. The
sequence generation code is mainly from the old load_PDB_sequence() function.
The Scientific Python data object atom_loop() generator method has been updated to allow for only
selected atoms to be yielded.
