Getters/setters for bond lengths, angles, dihedrals

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