relax SVN archive

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.

Changed the force flag of the structure.create_diff_tensor_pdb() user function to a bool.

The average CoM-pivot point vector is now labelled in PyMOL.

Changed the colour of the Average CoM-pivot point vector.

Modified the cone_pdb() function to decrease the bond widths.

Shifted structural_data from a class varible to an instance varible in the API base class.

This uses the __init__() function now.

Bug fix for the internal relax PDB data object.

The object structural_data inside the data object was a class variable, and hence each instantiation
of Internal_PDB was not resetting the object. If the object was used twice, then the data from the
first would remain.

Same bug fix as in r5412 but for tensor_pdb().