Added radio buttons to select histogram method for surface fields

[BernardClaxton]

Fixes #1173

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[BernardClaxton]

Fixes #1173 to allow users to select the method for computing histograms of surface fields. Possible methods are - frequency, normalised frequency, density.

[github-actions]

[jfrost-mo]

The overall design looks very sensible and it should provide a welcome improvement in our histogram plots.

I've left a few comments throughout, but the two big things are adding tests for the histogram function, and considering whether we are right in making this user configurable outside of the recipes. See this comment.

[jfrost-mo]

Sorry for being slow to review this. I'll go ahead and update the branch myself to fix the conflicting changes.

[BernardClaxton]

I've also been a bit busy James. I've tried to implement your suggested edits, but I've yet to write a test for the new histogram plotting function. I can also follow up on your suggestion to move the histogram method to the Setup.

[jfrost-mo]

Its looking good with the changes you have done. I've just brought the branch up-to-date with the trunk, so once we have a few more tests we will be ready to merge.

[daflack]

I'll rebase ahead of full review.

[daflack]

Looks good and works well. A really valuable change to CSET.

One very minor comment worth thinking about (also partly not sure if it was something I changed in the re-base that has altered it).

I was wondering if it was worth putting a little bit of information in the help text to indicate that the method selection appears in General Setup Options or seeing if there is a way it can be moved to appear in the location where the histograms are selected.

Also worth noting there should probably be a separate pull result to allow pressure levels and model level histograms to be selected in the same way as it currently does not trigger for those).

[BernardClaxton]

Originally the histogram method was selected where the histogram options appear. So yes, it might be worth popping this function back there.

This enhancement was designed to allow someone to select a single method for producing histograms. It might be worth considering allowing multiple methods to be selected, similar to how "MEAN", "MIN" and "MAX" can be selected for producing aggregated plots.

[jfrost-mo]

I'm tempted to suggest that we should add this as an argument to the histogram operator, and then use a recipe variable to set it instead of a command line option.

This will allow:

• Running multiple types in the same CSET workflow.
• Reproducing the same plot from a recipe.
• Allow configuring this from next to where the histogram options are.
• Allow hardcoding when a specific type is appropriate for a certain recipe.