Rationalization of the MMFF API before it's too late

- Replaced the setupMMFFForceField() function (which returned a pointer
  to a newly allocated MMFFMolProperties object) with a simple
  constructor of the MMFFMolProperties object
- Replaced in a few MMFF-related functions the "ROMol *" argument
  with a "ROMol &" argument for consistency with similar RDKit
  functions
- Renamed the SetupMMFFForceField() function in Python into
  GetMMFFMolProperties() for consistency
- Updated the MMFF tests according to the aforementioned changes
  in the API