Move prmtop instantiation outside of the setUp routine and put it in the module
itself. This way each parm is only read once for every test (rather than being
read for every test, even when they're not used, as part of the setUp routine).
For large systems, this makes the tests run a lot faster (for small systems it's
not a large speedup).

All tests continue to pass. This change also tests that the various calls to
createSystem and such do not actually change the object in a detrimental way.
Add NBFIX support to the Amber topology file parsers.  The new Amber ff14IPQ
force field utilizes nbfix functionality. Without this commit, using a ff14IPQ
topology file (or one modified by ParmEd to contain off-diagonal LJ elements),
would happily run, but ignore off-diagonal elements.

The new Chen and Garcia RNA force field (see doi:10.1073/pnas.1309392110) also
utilizes off-diagonal terms in its parametrization scheme.

One shortcoming is that the approach taken here -- using a pair parameter lookup
table -- cannot be currently used with a long-range correction because it is not
a `Continuous1DFunction' by virtue of using two `Discrete2DFunction'
TabulatedFunction classes.

Energies here match the values obtained with Amber to 0.01 -- 0.1 kcal/mol
(within 0.1% of the total).
Force the test to use the Reference platform (it's a tiny system, and all
TestAmberPrmtopFile.py tests take 17 seconds combined on my laptop).
Copyright update.
Merge pull request #524 from swails/nbfix_amber

Add support for Amber prmtop files with NBFIX