relax SVN archive

Removed the import of the now non-existant generic_fns.sequence.load_PDB_sequence() function.

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.

Merged generic_fns.sequence.load_PDB_sequence() into generic_fns.structure.main.load_spins().

Removed the call to load_PDB_sequence().

The main part of load_PDB_sequence() will shifted to this load_spins() function. Those parts
specific to the Scientific Python PDB representation will be shifted to the atom_loop() method of
the new scientific python data object.

Replaced the call to sequence_exists() by generic_fns.selection.exists_mol_res_spin_data().

Deletion of the sequence_exists() dummy function.

Deleted the load_seq arg unit test of the structure.read_pdb() user function.

The argument no longer exists.

Bug fix for the structure.read_pdb() user function.

Bad set up of the intro string.

Bug fix for the spin_id arg test of the structure.read_pdb() user function.

This now belongs to the structure.load_spins() user function.

Spun out the code for loading spins from read_pdb() to load_spins().