Allow overriding of interatomic potentials

[trisyoungs]

Describe the feature / issue
At present we auto-generate all of our pairwise LJ potentials from the supplied atom types and their parameters. This is convenient for the most part but is inflexible when one wishes to, for example, override a specific pair potential between unlike types with a different set of literature values, or to add on an additional term to an existing pair potential (in a similar spirit to EPSR's minimum distance potentials).