Changed the force constants so FENE bonds would work.
modified gromacs connection bonds to run.
Several modifications:

* Moved constraints to the abstract angle type.
* Added a ConnectionBond (gromacs bond type 5)
* Improved the logic for HarmonicPotential bonds.  Still a problem
  for LAMMPS, which does not construct the exclusions from the bonds right yet
  (Harmonic potentials are excluded from being used to create exclusions -- they
   are not 'chemical' bonds)
* Added FENE bonds