Replace 'config' directory with submodule

[ZedThree]

As discussed in #2397, moves all the configuration scripts out into a standalone repo and replaces them with a submodule to that repo.

[johnomotani]

Do we even want to include the configs as a subdirectory? Might be better to just document that the BOUT-configs repo exists?

I'm not sure, but if someone needs to edit some config file for their machine, wouldn't that make the BOUT-dev repo 'dirty' (in the sense of git describe)? I think it's nicer to be able to run with the repo being 'clean' as much as possible for provenance tracking purposes - actually it would be nice to record to the output/log files whether it was 'dirty' or not (and possibly even record the git diff - I think we discussed on Slack a while ago when I was doing similar things for STORM).

[ZedThree]

Very good points @johnomotani -- I agree, having the configs remain separate is probably the best thing to do.

@bendudson Any thoughts on this?

If we're happy keeping them separate, I'll remove the submodule from this PR.

[bendudson]

Thanks @ZedThree and @johnomotani yes I think this is a good plan. If anything I think the BOUT-dev repo could be a submodule of the configs repository: The configs are for a particular version of BOUT++.
We briefly discussed today having multiple config repositories e.g. NERSC, Livermore,... For now perhaps directories:

BOUT-config

• Cori/
  cori-config.sh
  ...
  BOUT-dev <- Submodule, pointing to known good commit?
• Perlmutter/
• Lassen/
• Archer/
• Marconi/

[ZedThree]

Bundling BOUT-dev in the configs repo is a good idea. That sort of layout especially makes sense if we want to go even further and make a "superproject" where we bundled everything together, something like:

|- BOUT-dev
|- configs
| |- archer
| |- cori
| ...
\- externalpackages
  |- netCDF
  |- HYPRE
  |- SUNDIALS
  |- PETSc
  ...