Forcefield constraints (distances, angles, torsions, positions)

- added relative DistanceConstraints (i.e., +/- with respect
  to the current value) (C++/Python)
- added absolute/relative AngleConstraints (C++/Python)
- added absolute/relative TorsionConstraints (C++/Python)
- added PositionConstraints (C++/Python)
- exposed fixedPoints from Python
- added relevant C++/Python tests
- removed a number of redundant "this->" in member functions
- moved some getGrad() code into Utils::calcAngleBendGrad and
  Utils::calcTorsionGrad to avoid repeating the same code
  for constraints