Fixes github issue #8597.
* UFFGetMoleculeForceField was returning
PyForceField* before the class was registered via rdForceField,
which caused “No Python class registered” errors in Python when
using multiple conformers.
* This patch forces rdForceField to be
imported during rdForceFieldHelpers module init, ensuring the
class is always registered before helper functions are used.
* Add unit test
cded6cc4e0