relax SVN archive

Bug fix, the 'self.sel_read()' function was not expecting the 'column' argment.

You can now specify a different column for residue numbers in the selection user functions.

This affects both the 'select.read()' and 'unselect.read()' user functions.

Addition of the user function 'structure.vector_dist()'.

This function will display the PDB representation of the XH vector distribution overlain over the
macromolecular structure within PyMOL.

Created a symmetric chain 'B' in the PDB representation of the XH vector distribution.

Added the ability to create ATOM PDB records within the 'self.write_pdb_file()' method.

Changed the format of the PDB representation of the XH vector distribution.

Each XH pair is being created as a proper vector with two atoms rather than all protons connecting
back to a single centre of mass.

Added the method 'self.create_vector_dist()' to 'generic_fns/structure.py'.

This function currently calculates and adds the centre of mass to the PDB file, adds the XH vectors
as protons to the 'self.atomic_data' data structure, and creates the PDB file. The code is
incomplete as each vector is being set to the true residue name and number and ATM record support
still needs to be added.

Started to created the 'structure.create_vector_dist()' user function.

This function will, when completed, create a PDB representation of the XH bond vectors from the
structure.

Spun out the XH bond vector calculation code into its own function 'self.xh_vector()'.

Renamed all instances of 'create_tensor_pdb' to 'create_diff_tensor_pdb'.