S20 Diags: geopot thickness

[Byron Blay (bblay-mo)]

PR Summary

Add Section 20 diagnostic thickness of geopotential height at two pressure levels, as described in https://github.com/MetOffice/Section20/issues/11.

Todo:

• Wait for Paul to make suites work with lfric apps vn3.0/GitHub
• I've discovered that we don't need the DOF loop in the kernel because we're processing lowest order W3 data. However, I don' t know if we can guarantee that we'll only see such data in the future. I need to either a) discuss this with someone who knows the use cases / problem domain, or just leave the loop in to be future-proof - but does that incur a significant overhead?
• Are the standard name and long name correct?
• [How] do we check the new fields are correct using KGOs?
  • The kgo update didn't do anything. I assume we need to wait for the suite upgrade.
• I can't remember why the thickness fields are initialised to 1.0 before the kernel is called, and not 0.0.

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[Byron Blay (bblay-mo)]

Replacing with #454, rebased onto stable.