Created functions for calculating the center of mass of the selected residues.
The function 'self.center_of_mass()' in the file 'generic_fns/pdb.py' has been created to calculate
the center of mass. All unselected residues are skipped to allow hybrid models, two fused runs of
non-overlapping residues, to be handled. Hence the center of mass will be biased!
The function 'self.atomic_mass()' has also been added to the file. This function returns the atomic
mass corresponding to the supplied element string.
