Convert to git-fleximod

[gold2718]

git-fleximod is a replacement to manage_externals

• Developed at NCAR: https://github.com/ESMCI/git-fleximod
• ./bin/get-fleximod update replaces ./manage_externals/checkout_externals

[MichaelSchulzMETNO]

Just for info: Had problems with cloning a recent 2.5 CAM repository with fleximod using Github desktop, it worked only on the terminal.

[TomasTorsvik]

I also made an announcement under the discussions page (#599) since some people may look there first, but I suggest to keep this issue open to discuss potential problems that we encounter.