Add support for additional functional forms for vdW interactions

[bannanc]

This is a duplicate of smarty issue#161, but we are trying to get all relevant issues here instead of in the smarty repository when possible.

We currently only support Lennard-Jones interactions, but it would be easy to generalize NonbondedForce beyond this to support Morse, Buckingham exp-6, Halgren buffered 14-7, and other functional forms.

Suggestions in https://github.com/open-forcefield-group/smarty/issues/221

• the generalized Lennard-Jones (m-n) potential
• the Buckingham exp-6 potential

[jchodera]

We have extended the SMIRNOFF spec to support this via the vdW section, though we have yet to implement support for functional forms beyond Lennard-Jones 12-6 in the toolkit.

I'll leave this issue open to remind us to implement additional functional form support.

[mattwthompson]

ref https://github.com/openforcefield/standards/issues/46