CESM moving from use of manage_externals to "git submodules" in cesm3_0_beta01

[ekluzek]

CESM is considering moving from the use of manage_externals to using "git submodules". The CTSM PR that will do this for CTSM is here:

ESCOMP/CTSM#2443

Since, the mizuRoute externals are only used for standalone cases it doesn't need to happen the same time as other CESM components. But, should likely be done fairly soon afterwards as manage_externals is already NOT being supported at this point.

[ekluzek]

This is happening in roughly a month. So we'll need a PR to do this in mizuRoute. We can check with Jim if he is willing to do it, or if we will need to do it. I think it's not too hard either way.

[ekluzek]

Since, for mizuRoute we only have one external, and it's only needed for offline mizuRoute doing this does NOT need to happen immediately. So when we have a chance we'll update this for offline mizuRoute, but doing this isn't tied to any CESM timelines.

[ekluzek]

cesm3_0_beta01 has been made where the move to git-fleximod has been done for all other CESM components. So we should now bring this in for mizuRoute.

• Add "git subtree" for git-fleximod as in other CESM components
• Update README_EXTERNALS file to mention git-fleximod
• Update Makefile that mentioned manage_externals
• Put above into a PR and merge to cesm-coupling
• Test that git-fleximod functions as we want
• Bring in remove manage_externals #468 that removes manage_externals