Add EarthWorks Diagnostics as an external

[gdicker1]

This PR incorporates the initial version of another important part of the EarthWorks project: our analysis scripts. Currently this external only contains scripts that produce plots for FHS94 and FKESSLER compsets.

[gdicker1]

A couple of things with this one:

• First I put this external into a folder under the EarthWorks repo "tools/ewms-diags"
• It's fine to use the main branch for now, but we should change this to a tag at some point in the future. Both as a best practice and to match with our other externals
• I set this as a required checkout, but that was an arbitrary choice. I could see arguments for either setting.

@areanddee @sherimickelson , let me know what you think. Particularly about the external name and location!

[areanddee]

Thanks for adding the diags. Looks fine to me, with no conflicts (new stuff).