Use better data structures in gmx chi

Use better data structures in gmx chi

This removes part of the dependency of gmx chi histogramming from
ResidueType handling, which will permit changes to the latter that
will benefit both grompp and pdb2gmx

Replaced vectors that needed a stable numerical index to look up a
residue name with a map that uses the residue name to do the look up.

Removed capabilities of ResidueType that are no longer used.