Fix CIF rhombohedral symmetry scoring mutating imported lattice

[clemisch]

Score alternate CIF space-group settings on a temporary Crystal instead of the live imported object, preventing :R/:H probing from corrupting rhombohedral hexagonal cell metrics during pyobjcryst CIF import.

See #67

[vincefn]

Thanks for the PR - I'm not sure creating a tmp Crystal object is the best approach. It creates a new entry in the global Crystal registry, which should not be necessary.

Could this be done instead by just creating a tmp SpaceGroup object, or even better only using a sgtbx call ?

[clemisch]

Updated the fix to avoid constructing a temporary Crystal during _symmetry_equiv_pos_as_xyz scoring. The candidate setting is now evaluated directly with cctbx::sgtbx::space_group. The live Crystal is only changed once at the end with the selected canonical symbol.

Is that what you meant in your suggestion?

One caveat: the candidate-parse path now uses catch(exception) only to preserve the old "skip invalid candidate setting" behavior with the sgtbx parser. That is broader than ideal and should be narrowed to the specific cctbx/sgtbx exception type.

[clemisch]

I think I found the appropriate exception type: cctbx::error