Fix externals reference for GSI in Externals.cfg

[christopherwharrop-noaa]

DESCRIPTION OF CHANGES:

The ncdiag was removed from GSI and replaced by a submodule. This requires the GSI to be checked out with the ncdiag submodule, which requires addition of a separate Externals_gsi.cfg file to specify which GSI submodules are needed. Unfortunately, there is no other reasonable way to accomplish this without adding the Externals_gsi.cfg file because the manage externals utility does not allow for specification of specific submodules at the top level. Additionally, GSI will not build without ncdiag. So, even though there exists an option to turn off the building of ncdiag, that doesn't help because GSI will not build unless ncdiag is also built. This is the cleanest solution available.

TESTS CONDUCTED:

This change only affects the build, so only a build test was done. The App was cloned, externals were checked out, and the code was built, on Jet. The ./manage_externals/checkout_externals now works correctly and checks out the ncdiag submodule of GSI.

DEPENDENCIES:

N/A

DOCUMENTATION:

None needed.

[hu5970]

@christopherwharrop-noaa This is a fix we needed for compiling GSI. Do we know why FV3LAM module can checkout all submodules but GSI cannot without adding this new Externals_gsi.cfg?

Let me try to delete "externals = None" and try to checkout GSI.

[christopherwharrop-noaa]

@hu5970 - The externals = None was done to prevent "manage_externals" from trying to check out the GSI fix file submodule which is hosted on VLab. That was before ncdiag was made into a submodule. If you remove externals = None then when you run the ./manage_externals/checkout_externals command it will try to check out all submodules, both ncdiag and fix. That will work iff the user has access to VLab and the GSI fix file repository and has the correct gerrit configuration set up in their ssh config. So, this PR fixes the problem by allowing anyone to check out GSI and its ncdiag submodule.

[hu5970]

@christopherwharrop-noaa I actually has removed the fix as submodule in NOAA-GSL/GSI rrfs_dev branch because we don't need this fix directory. Adding "Externals_gsi.cfg" to control is a better way to go.