This changes the CSV output file to list one model ligand per line,
instead of one reference ligand.

Added script-level info on progress; warnings upon crazy chain mappings;
silenced Verbose logging about selections; generally increased verbosity.

QS-score which is used as default target function for chain mappings.
However, thats protein specific. As soon as nucleotides are involved,
lDDT with an increased inclusion radius of 30A is now used
in ChainMapper.GetMapping.

It was a bit embarassing that lDDT mappings were about an order of
magnitude slower than QS-score mappings. A specialized backbone only
lDDT has therefore been introduced that uses matrix operations.
This is not a replacement of the lDDTScorer. But it doesnt need to
produce per-residue scores and doesn't need to deal with symmetries
etc.

and some cleanup...

This is no final product but just a first working version as a
starting point. Plenty of testing/cleanup/speedup required.

Until now, the actions were failing with somewhat cryptic messages
raised from arbitrary locations. This one should be more user-friendly.

CreateBU always returns a valid entity. We already check if it's empty
separately.

This is made possible now that we have a CreateBU function.

It turns out for some reason we were reading them from chains, based on
the ChainType of the chain. This prevented extracting anything from
legacy PDB files and didn't match the documented behavior in the action.

Now extraction is based on the IsLigand flag only, and documentation is
consistent throughout. Behavior on PDB files is tested.

Move long description to separate properties

Yolo, this code is totally untested. Xavier, please give it a try ;)

Following fixes in cd701347, setting seqres=True is no longer required
for these things to work.