Defining a Foyer XML format

[mattwthompson]

There are now a few force field XML formats: the original OpenMM XML, Foyer's soft fork, Open Force Field's flavor (https://github.com/openforcefield/openforcefields/tree/master/openforcefields/offxml), and another we are developing in the backend. To somewhat clarify compatibility, I propose we introduce a Foyer-specific file extension that explicitly tells a loader that a given XML is meant to be a Foyer XML. This can be .foyerxml, .fxml, or something else, I don't have a strong preference. This would involve

• Modifying the loaders to prefer this extension and possibly warn if file.xml is passed (i.e. "Reading file in as a Foyer XML, consider changing the file extension to .foyerxml")
• Not erroring out if a file.xml is passed, thus making this a non-breaking change
• Changing some of our own XML files to this new extension
• Updating documentation specifying the differences between this and other XML format(s)

What do you think @mosdef-hub/mosdef-contributors?

[mikemhenry]

Sounds reasonable, .ffxml might be better for SEO someday so we don't conflict with fxml. We could also have a tag like this <foyer_xml version="0.7"> so that we can track changes we make to our schema and know what tags to expect when parsing.