Handle version in SMIRNOFF XML files (and renaming from SMIRFF)

[bannanc]

There are variety of lines in the SMIRNOFF FFXML files that have the word SMIRFF, such as the following:

<SMIRFF version="0.1" aromaticity_model="OEAroModel_MDL">

These should presumably changed to SMIRNOFF, but I didn't want to make changes that might break the forcefield code. Also the version isn't really for SMIRNOFF, it should be for SMIRNOFF99Frosst, I think.

[davidlmobley]

I think the intent is that these tags be tied to a SMIRNOFF version spec, specifying which version of the SMIRNOFF format is used, is that right, @jchodera ?

How, ultimately, will we achieve this, @jchodera ? Does it mean that future code changes to ForceField need to maintain backwards compatibility somehow?

[davidlmobley]

I think I got the names changed to SMIRNOFF from SMIRFF. However, still need input from @jchodera on how we're handling versioning of the spec.