relax SVN archive

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().

Updated the structure.read_pdb() user function.

The arguments load_seq and spin_id have been removed as this functionality has been shifted into the
new user function structure.load_spins().

Created the structure.load_spins() user function.

The back end does not exist yet.