Fix LRU cache for ray and update 'min_atoms' option

The ray actors were failing to properly import the cython code in the lru-dict library. Kept giving the error AttributeError: type object 'LRU' has no attribute '__len__'. Switching to pylru (pure python) fixed the issues.

Also updated the builder and molecule state objects to respect the min_atoms parameter meaning no terminal state will be an available action until at least min_atoms heavy atoms are present in the molecule.