relax SVN archive

Fix for the centre_of_mass() function.

When no element entry is present in the PDB file, a RelaxError is thrown.

Bug fix for the centre_of_mass() function.

The spin id string is now created to correctly return the molecule, residue, and spin containers.

Removed the PyMOL execution commands from the XH bond distribution PDB representation system test.

Another bug fix for the create_vector_dist() function.

This time the atom number was not being incremented for chain B so that there are multiple atoms of
the same number in the PDB file.

Bug fix for the create_vector_dist() function.

The chain B of symmetry vectors was exactly the same as chain A and not mirrored.

Bug fixes for the PyMOL cartoon() method.

As with the open_pdb() method, cartoon() was not handling the structural object correctly and was
not handling multiple structures.

Bug fixes for the PyMOL open_pdb() method.

This method was not handling the structural object properly.

Bug fix for the create_vector_dist() function.

A structure needed to be added to the internal structural object.

Converted the symmetry flag to a bool.

A number of fixes for the create_vector_dist() function.