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