XCH4 ESA CMUG diagnostics (subset of the MPQB diagnostics)

[hb326]

First diagnostic to use ESA CMUG XCH4 data for evaluating CMIP datasets.
Diagnostics are a subset of the C3S MPQB diagnostics, some with minor adjustments.

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This pull request needs ESMValCore pull request 1428 (ESMValGroup/ESMValCore#1428) to be merged to be able to run the recipe.

[hb326]

• I have commented in the observational dataset.
• I cannot think of another reference for these diagnostics. They are very basic in the sense that they are just lineplots without too much statistics behind it...
• I adjusted the provenance, so that it states next to "global" also "nh" and "sh"
• It looks to me like "cmug" is already added as project in the provenance. At least it is listed in the provenance record where it is created. Hmm, there might be something going wrong...
• I changed the caption in the provenance records for the growth to "global mean annual growth rates" as suggested
• I adjusted the "statistics" for the growth rates plot
• I added a section to the documentation about the observations and the derivation of the variable

[axel-lauer]

@hb326 Thanks for working hard on this! I think most of the point I raised earlier are fine now. What I still fine a bit strange is the following in the provenance:

• the attribute 'domains' now always says 'global, northern hemisphere, southern hemisphere' instead of just one of the latter
• the attribute 'caption' continues to say "global ..." instead of giving the specific region

Other than that, I think all looks good! Nice work!

[axel-lauer]

Receipe runs fine, documentation and output look good!

[schlunma]

Cool stuff! 🎉

I only have minor comments regarding the documentation, commented lines and specific plot labels.

[axel-lauer]

@schlunma Thanks for your review! @hb326 I would recommend to accept all of @schlunma 's proposed changes. Then the PR can be merged. Yay!

[hb326]

You were a little quick, @axel-lauer, with the merging. :)
I wanted to update the recipe figure because it is without the observations right now. Will do that in a new pull request then...

[axel-lauer]

@hb326 ups... Sorry about this. I created a pull request with an updated figure including the ESACCI data: #2502
Please take a look and approve PR if you are happy.