Reorder methods as in the base class.

No change in code function.
Implement diff method for NoSymmetryStructureAdapter.

Calculate diff using the wrapped objects.
Make StructureAdapter::totalOccupancy non-virtual.
Refactor usefullsum flag storage to a bit flag.

This permits simple expansion to multiple configuration flags
for PQEvaluator objects.
Add PQEvaluator flag FIXEDSITEINDEX.

When active, fast value updates are only possible if unchanged atom
sites remain at the same site indices in the new structure.
Also add descriptive comments for PQEvaluatorFlag-s.
Support USEFULLSUM in PQEvaluatorOptimized.

USEFULLSUM was assumed false till now.
Support fast-updates for BondCalculator.
Use set difference and merge to speed up BC.
Stash value by swapping instead of copying.
Optimize splitting to chunks.
Use local variable entry for readability.