Ensure that pdb is always uppercase

PDB ids are meant to always be uppercase so we make sure of that when generating
the ids. This may also be true for residues but I need to check that.
Remove extra assignment to residue

We should only set residue once but it was set twice.
Add code to map from experimental to resolved

Not all residues which went into the experiment are resolved. We need to know
the mapping between experimental and resolved for many different things. This
adds code to compute such a mapping using the pdbx_poly_seq_scheme table as
suggested by John. We now can get both the experimental sequence as well as a
mapping between a sequence id and the corresponding unit id. The sequence is
still in flux but this is a good start. We should also add some tests to make
sure the unit ids are generated correctly in all cases.
Add an example program for getting  mapping

That is useful enough that we should have an example of how to use the code to
get such a mapping.