A few fixes to improve MMFF/UFF robustness

- fixed a bug in Code/ForceField/MMFF/testMMFFForceField.cpp
- fixed a bug in Code/GraphMol/ForceFieldHelpers/MMFF/AtomTyper.cpp
  which caused misassignment of atom types in CYGUAN01 upon shuffling
  the order of atoms in the validation SDF files
- changed hard-coded constants into const
- added checks for acos and asin function parameters to be within
  a (-1, 1) range