Replacing checkout_externals with git-fleximod

[TomasTorsvik]

The latest tag noresm2_5_alpha08 replaces checkout_externals with git-fleximod. This means, when checking out source code, you should not run the checkout_externals script, but instead run

./bin/git-fleximod update --optional

• See ./bin/git-fleximod --help for further info and options.
• Developed at NCAR: https://github.com/ESMCI/git-fleximod

Related to this change, external components are now defined in the .gitmodules files instead of the Externals.cfg file.

Issues related to git-fleximid should be reported and discussed under issue #581

[TomasTorsvik]

Updating components managed by git-fleximod

Updating a NorESM component tag has two goals:

• Commit NorESM (top level) so that all component submodules are recorded in the desired state. That is, running git checkout --recurse-submodules <new_tag> at the top level should checkout the NorESM tree to the correct state.
• Document the correct tag(s) by updating the fxtag entries in the top-level .gitmodules file

This procedure should always work:

1. In your NorESM clone, create a new update-BLOM-tag branch and fully check it out (git checkout -b update-blom-tag NorESM/noresm_develop && ./bin/git-fleximod update --optional).
   • git submodule status should be 'clean' (the first column is blank for all submodules).
2. In components/blom, checkout the new tag and run git submodule update.
   • git submodule status should be clean.
3. At the top level, update the fxtag value in the blom submodule section of the .gitmodules file with the new BLOM tag.
4. Run git commit -a -m 'Updated BLOM tag to rev.x.y.z.q'.
5. Push this to your fork and open a PR.