UFF hack for 3- and 4-membered rings containing sp2 atoms

- Implemented a hack to improve the UFF-generated geometries
  of 3- and 4-membered rings containing sp2 atoms. The hack
  consists in altering on-the-fly the theta0 equilibrium angle,
  depending on ring size and collocation of the two edges of the
  angle (i.e., both edges inside the ring or one inside and one
  outside)
- Added a relevant test in
  Code/GraphMol/ForceFieldHelpers/UFF/testUFFHelpers.cpp