Resolve gmxapi versioning with respect to GROMACS releases and packaging

[eirrgang]

gmxapi 0.0.8 and GROMACS 2020 probably won't be compatible with 0.0.7 client code.

According to https://redmine.gromacs.org/issues/2764 GROMACS will probably change the reported compatibility in gmxapi-config.cmake from SameMajorVersion to ExactVersion, making it the client's responsibility to determine compatibility.

kassonlab/gmxapi and kassonlab/sample_restraint can be updated to check for explicit gmxapi versions in reverse chronological order until we have a clearer compatibility guarantee. We can issue "tweak" releases on the release-0.0.7 branch to handle necessary compatibility tweaks.

Update: All gmxapi software for API levels > 0.0.7 is officially maintained in the main GROMACS repository. The current repository (as well as kassonlab/gromacs-gmxapi and gmxapi/gmxapi) will undergo some rearrangement to tie 0.0.7 software to GROMACS 2019 while keeping these GitHub repositories pointing to something useful.

• Equate gmxapi 0.0.7 with GROMACS 2019. (PR Tie gmxapi 0.0.7 to GROMACS 2019 #225)
• Update docs and merge gmxapi devel branch to master.
• Bump 0.0.7 patch release number and merge to release-0_0_7.
• Replace kassonlab/gmxapi master branch with the Kasson Lab GROMACS fork (the sandbox-gmxapi branch from GROMACS Gerrit repository).
• Update docs at kassonlab/gromacs-gmxapi to clarify that it is an old development repository; users should go to kassonlab/gmxapi, GROMACS 2019, or GROMACS 2020+ depending on what they are looking for.
• Update GROMACS python_packaging/documentation/ or https://gerrit.gromacs.org/c/gromacs/+/11605 when rearranging repositories.
• Update kassonlab/sample_restraint to a stable referent of gmxapi 0.0.7 / GROMACS 2019 with pointers to GROMACS 2020+.

[eirrgang]

We can allow that 0.0.7 only works with GROMACS 2019 and make this issue pertain only to versions 0.0.8 and higher.

[eirrgang]

gmxapi development is moving to the GROMACS repository. This repository will continue to house gmxapi packages through 0.0.7.x, which will be tied to GROMACS 2019. I am updating the development branch and planning to merge it finally to 'master'. The 'release-0.0.7' branch will be allowed to lag behind master for stability, but merges will be staged directly against 'master' from now on.