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This was changed to help debug my development.  I have now returned it
to the original behavior.

This should fix CI build errors in parent repository.

The old _is_gro_coord function inexplicably used split() even though
gro files fixed field (nb: the parsing was still done correctly).  Now
it uses fixed field substrings.

I have made things more technically correct: GROMACS processes pairs
and exclusions separately.  Also added proper support for fudgeLJ term.
Also added improved gro file parsing (technically it’s a fixed-field
format but this works too).

These lines were missed previously

This is the initial commit of my work in getting OpenMM GROMACS support
improved to the point where SMOG SBM simulations run out-of-the-box.
Further commits will be code cleanup.

- Make Python 2.7 a minimum requirement per pandegroup/openmm#1656
- Make the bond classes as well as other forcefield.py singletons inherit from
  Singleton to make them pickleable
- Make class Bond inherit from namedtuple instead of tuple