This is ready for review and comment. I will leave it open for at least 24 hours, merging no earlier than Mon 28 Dec 14:00 GMT (15:00 local).

NB: This particular issue is blocking the next release of the OPS CLI (which will include SimStore support!) and so I'll probably go ahead and release a 1.4.1 shortly after this is in.

LGTM, 1 quick question:
I don't see where pip install openpathsampling[simstore] would install simtk? (I only see sqlalchemy and dill added to the cfg)

Is simtk only a testing requirement for SimStore or did you forget to add it to the cfg? (just making sure it was intentional, I am not too familiar with the internals of simstore anymore)

Is simtk only a testing requirement for SimStore or did you forget to add it to the cfg? (just making sure it was intentional, I am not too familiar with the internals of simstore anymore)

Sorry, the PR message wasn't clear. Two separate issues here:

1. Using SimStore should not require that simtk.unit is present in the environment (you should be able to use and test the new storage without OpenMM installed). But you couldn't use experimental.Storage without importing simtk.unit -- that's the main thing this PR fixes.

2. As a separate issue, currently I say "if you want to use SimStore, you need to install a couple extra packages..." -- adding the simstore extra in setup.cfg makes that process a little more elegant (at least, in some use cases).

Sorry, the PR message wasn't clear. Two separate issues here:

Using SimStore should not require that simtk.unit is present in the environment (you should be able to use and test the new storage without OpenMM installed). But you couldn't use experimental.Storage without importing simtk.unit -- that's the main thing this PR fixes.

As a separate issue, currently I say "if you want to use SimStore, you need to install a couple extra packages..." -- adding the simstore extra in setup.cfg makes that process a little more elegant (at least, in some use cases).

Ah, thanks for the clarification! All looks good to me then :)