S20 Diags: snow prob

[bblay-mo]

PR Summary

Add Section 20 diagnostic snow probability, as described in https://github.com/MetOffice/Section20/issues/21.1

Todo:

• Just discovered, we don't need to iterate DOFs for 2D fields. Remove the DOF loop.
• Wait for Paul to make suites work with lfric apps vn3.0/GitHub
• Is the long name correct? What about a unit?
• Rebase once S20 Diags: geopot thickness #98, from which this was branched, has been merged.
  • Until then, this PR will also show the changes from S20 Diags: geopot thickness #98.
  • We can see just the changes ontop here (needs fork invite): https://github.com/bblay-mo/lfric_apps/compare/diags_geopot_thickness...bblay-mo:lfric_apps:diags_snow_prob?expand=1

Sci/Tech Reviewer:
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trac.log

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