Add example/cookbook/tutorial demonstrating ways to create/modify a ForceField

[mattwthompson]

Off the top of my head, one can

• Write a bunch of OFFXML files/snippets
• Add handlers and/or parameters via objects
• Add handlers and/or parameters via kwarg/dict blobs
• Do any combination of the above, appending to an existing force field(s) on-the-fly
• Creating custom parameters, but only appending to an existing force field after all are created

There's a lot here to communicate, and I don't think there's a central location that shows each of these routes side-by-side. I find creating custom parameters somewhat confusing, still, after years of mucking around in this.

An existing example touches on this but does not serve the purpose I'm after, which in plain English is "I don't want to explain this in a more involved example - I want to link to an example that shows how to do this without added complications."

This would fall under the toolkit's domain - feel free to transfer this issue if desired.