Add XML for 3 body potential

[rsdefever]

Add a simple XML file for a mW water. This is to make sure GMSO can handle 3-body potentials internally. At this stage I don't imagine implementing a SW writer for LAMMPS.

[mattwthompson]

We discussed this briefly in person, but just to put some general ideas to paper -

• Supporting new potentials is a task for both the XML schema and some internal class. If a Stillinger Weber potential can be implemented without making a new Potential subclass, cool. If it needs one, that will need to be written.

• In this case I'm not sure if you want to start specific (SW) or more general (some more generic class for three-body interactions).

• Ideally, the in-memory (class) and on-disk (XML) representation have major similarities since they are inter-dependent in most any relevant use case.

• Writing engine interoperability is definitely not necessary to get a new feature merged, nor is it necessary to claim we can support it. But it's something that should be considered in design, i.e. ensuring that the XML and in-memory representations of a Stillinger Weber potential are not so specific to the implementation in LAMMPS that they would need to be heavily refactored to work in a different engine.