To-do items for Github Actions CI

[leeping]

• EITHER putting the OE license back, OR changing the little remaining functionality that requires it over to SDF (should be possible now that OFFTK supports partial-charges-in-SDF). I'd think that the latter is a better choice since it will remove the need for an OE license in CI and will let @leeping allow PRs from forks.
• Re-enabling Py27 tests
• Re-establishing double precision GROMACS builds

[mattwthompson]

Some folks have move GROMACS over from Bioconda to Conda Forge. I have a feedstock up that adds double-precision builds - it should be as simple as specifying it in the environment file.