Getters/setters for bond lengths, angles, dihedrals

 - added getters/setters for bond lengths, angles, dihedrals
   in the MolTransforms namespace:

  * getBondLength(conf, iAtomId, jAtomId)
                 (only in this case, atoms need not be covalently bonded)
  * setBondLength(conf, iAtomId, jAtomId, value)
  * getAngleRad(conf, iAtomId, jAtomId, kAtomId)
  * setAngleRad(conf, iAtomId, jAtomId, kAtomId, value)
  * getAngleDeg(conf, iAtomId, jAtomId, kAtomId)
  * setAngleDeg(conf, iAtomId, jAtomId, kAtomId, value)
  * getDihedralRad(conf, iAtomId, jAtomId, kAtomId, lAtomId)
  * setDihedralRad(conf, iAtomId, jAtomId, kAtomId, lAtomId, value)
  * getDihedralDeg(conf, iAtomId, jAtomId, kAtomId, lAtomId)
  * setDihedralDeg(conf, iAtomId, jAtomId, kAtomId, lAtomId, value)

- added the corresponding Python wrappers in rdMolTransforms
  (same prototype as C++; as usual all Python methods
  have an uppercase initial)

- added relevant C++/Python tests