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Adds a test for it as well.

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).