Update column names to be more informative

[jchodera]

Now that we're distributing FreeSolv in multiple forms (SDF files, Orion datasets), it would be useful if the column names (that become SD tags) were more informative and self-documenting about things like units. For example, the "expt" label is problematic since (1) it doesn't describe which of the many properties this is the experimental value for, and (2) it doesn't give the units. More informative names would very much help!

[davidlmobley]

Agreed! What's your favorite way to attach units now? simtk.unit, or have you migrated towards something else?

Though, there's also the issue of how things will pickle or go to JSON which I've not thought through yet. Perhaps we should just embed units in the name of the key or tag.

[jchodera]

We're still using simtk.unit for Python, which can also render units in a human-readable form.

@bas-rustenburg has used pint, which is very similar and independent of OpenMM.