Merge pull request #5473 from RosettaCommons/roccomoretti/interactive_fix_rdkit_rotamers
[Foldit] Fix ThreadedRDKitRotamerLibrarySpecification
The merge with master resulted in a crash with ligand design puzzles, due to the need of using the clone() method (which was unimplemented before, due to uncopyable mutex. By moving the thread-related code to a dedicated helper class, we can make the ThreadedRDKitRotamerLibrarySpecification::clone() method functional. (Clones share the thread-using class.)