Added function that calculates the distance from a point in cv space.

The point in CV space can be specified using a pdb file, hence the changes
to Action, Value, etc - all these changes are all so that you can calculate CVs from a
pdb input file.  Whilst I was messing about I noticed a potential problem with
natural units and input pdb files.  As we are in natural units there is no
way of knowing how to convert the pdb from angstroms (unit in pdb)
to the natural length unit.  For these reasons I assume that when one is in
natural units pdb input files are also in natural units, this required changes
in ColvarRMSD and my new ColvarTarget routine as well as a small change in Atom.h.