- removed a few spurious, though non harmful,  "const" keywords from
  Code/ForceField/MMFF/Params.h, Code/ForceField/UFF/Params.h,
  Code/GraphMol/ForceFieldHelpers/MMFF/AtomTyper.cpp
  and Code/GraphMol/ForceFieldHelpers/MMFF/AtomTyper.h (I realized
  their uselessness thanks to a warning issued by Intel C++ compiler)
- refactored O3A code
- added the possibility to set weighted constraints on selected
  atom pairs
- added an option to carry out local-only optimization
Merge pull request #250 from ptosco/master

O3A refactoring and new features