S20 Diags: geopot thickness

[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 addition to rose-stem/app/lfric_atm/file/file_def_diags_oper_nwp_gl.xml?
  • Add a KGO? FInd out how.
• 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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