GitLab

- 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

The previous version of this code would compute
the values of d0 and m0 for every iteration. This
was unnecessary as these values are fixed at the
start of the calculation by the radii of the particles
involved. This resulted in unnecessary computation
and memory access that dramatically slowed simulations
using the GBn or GBn2 solvation models.

The new version pre-computes the values based on the
radii that are present in the system. Linear interpolation
is used, which is consistent with Amber. The previous
version of OpenMM used cubic, which gives slightly
different results.

This doesn't lead to incorrect results, but it does erroneously flag some radii
as being illegal when they aren't (making it impossible to run some suggested
systems).

The parameters need to be compared to the table limits *before* subtracting off
the offset, whereas it's currently being done after.