refactor rigid scoring - introduce GDT implementation similar to LGA

The Scorer object provides two mappings. The default mapping which is
QS-score based and the rigid_mapping which is RMSD based.
The rigid_mapping is now the basis for global superposition based
scores such as RMSD and GDT. And here's the thing, OpenStructure got
a brand new GDT implementation! Scanning of best possible superpositions
is a bit simpler than in LGA but the results are similar.
Benchmarking against CASP15 TS models returns GDT_TS score where 99.2%
are within 3 GDT points with the LGA results and the maximum observed
difference is 7.39. There is a tendency for slightly lower scores in
OpenStructure meaning LGA sometimes found better superpositions.

BUT: Oligo/RNA support comes for free in the Scorer object!