CESM has moved away from manage_externals to the use of "git submodule" and git-fleximod

[ekluzek]

CESM is considering moving from the use of manage_externlas to using "git submodules". The CTSM PR that suggests a way to do this for CTSM is here:

ESCOMP/CTSM#2303

If CESM moves to this form, we would do the same for SLIM. @jedwards4b is willing to do a PR for mizuRoute similar to the other components when that decision is made for CESM.