Add git-fleximod ci test

[peverwhee]

Updates git-fleximod to v8.4 and adds the git-fleximod github workflow.

Describe any changes made to build system: N/A

Describe any changes made to the namelist: N/A

List any changes to the defaults for the boundary datasets: N/A

Describe any substantial timing or memory changes: N/A

Code reviewed by: cacraigucar, mwaxmonsky, nusbaume

List all existing files that have been modified, old files eliminated, new files added and describe the changes:

A .github/workflows/fleximod_test.yaml

derecho/intel/aux_cam: all tests pass
derecho/gnu/aux_cam: all tests pass

Summarize any changes to answers, i.e.,

what code configurations: N/A
what platforms/compilers: N/A
nature of change (roundoff; larger than roundoff but same climate; new climate): N/A

[mwaxmonsky]

Looks great @peverwhee! Just one minor comment but nothing that should hold this up.

[cacraigucar]

This appears to be identical to what we brought into ESCOMP/CAM, so I am approving this. I figure if there are any issues, they would show up when it is committed.

[nusbaume]

Should we add some text to the new CAM-SIMA developer documentation that describes (or includes a link to a description of) these tests? That way we can point developers to something if the tests fail.

[peverwhee]

@nusbaume yes! done!

https://escomp.github.io/CAM-SIMA-docs/development/cam-testing/#git-fleximod-tests

[nusbaume]

Thanks for adding the documentation! Just had a couple more suggestions.

[nusbaume]

Looks good to me now. Thanks @peverwhee!