and some cleanup...

we differentiate between pair states, i.e. errors related to a specific
pair of model/target ligands, and single ligand states, i.e. errors
That affect one particular ligand. An error state in the latter basically
takes that ligand out of the race for assignment.

This is mainly for debug purposes

That was stupid before

Only initial work done for a base scoring class that takes care
of ligand prep, computing ligand symmetries and calling score computation
functionality from child classes. Child class for SCRMSD is already there
with basic testing.

Can occur in chain_mapping._ChainMappings that provides an iterator
of all possible ways to map mdl chains onto fixed ref chains.

The situation where a chem group (group of chains in the reference) is
empty but the respective group of chains in the model is not can be
correctly resolved.