Separate units package

[mattwthompson]

In some recent threads (openmm/openmm#2997, openmm/openmm#3002, likely many others) the idea of having a separate simtk-units/openmm-units package was proposed and met with favorable responses. An obvious use case, if somewhat niche, is installing the units module without the rest of the OpenMM API and compute layers. I've actually run across this recently in a toy repo, so I would benefit somewhat from this.

I am happy to contribute to or do the implementation of this if provided some guidance. I don't know if refactoring the current recipe to use outputs: makes sense, or if simply contributing a new package to staged-recipes is a better approach.

P.S. I'm only concerned with the conda packaging here, but it's worth pointing out that that is an ancient PyPI project that is several years out of maintenance that could provide some confusion if people haven't forgotten about it and happen to try using pip to install it: https://pypi.org/project/simtk.unit/

[jaimergp]

outputs would be the way to go if the source is going to stay in the same repo. I'd wait until next major release, where simtk->openmm is done, to start the split packaging, though.

[jchodera]

It might be useful to convene a brief developers chat with @peastman as well to see if there might be a good path to adding interoperability support for standard unit systems like pint as well, so that we can at least support pint as well as simtk.unit units.