Added radio buttons to select histogram method for surface fields #1173

[BernardClaxton]

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• Documentation has been updated to reflect change.
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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.