MMFF branch created

Created a MMFF branch. The only change to the existing master
is that I moved the UFF Wrap and test files/folders from ForceField
and ForceFieldHelpers to the UFF subfolder; it seemed to me a
more appropriate location since from now on UFF will not be the
only force field available in the RDKit. I also made minor changes
to CmakeLists.txt files to reflect the different localizations of those
files/folders.

Paolo