Add esmvaltool compare command to compare the results of two recipe runs

[alistairsellar]

Description

Add an esmvaltool compare command to compare the results of two recipes runs, exposing the functionality of esmvaltool/utils/testing/regression/compare.py on the command line.

Closes #2381

Link to documentation: TBA

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[alistairsellar]

Hi @bouweandela, here is a draft of how the new esmvaltool compare command could work. Early comments welcome.

It runs successfully for me, in an esmvaltool environment. However, GitHub tests fail, I think because esmvaltool is not available for esmvalcore testing (E ModuleNotFoundError: No module named 'esmvaltool'). This would make sense, because you want to be able to test esmvalcore standalone without a dependency on esmvaltool.

If I've understood that correctly, would you have any concerns about me moving compare.py from the esmvaltool repository to esmvalcore?

Also, I'd welcome your opinion on what tests should be added for a new command, and where/whether documentation should be updated. On the second question, as far as I can tell, some commands are documented in the "getting started" section. This compare command could be described at the end of https://docs.esmvaltool.org/projects/ESMValCore/en/latest/quickstart/output.html?

[bouweandela]

It runs successfully for me, in an esmvaltool environment. However, GitHub tests fail, I think because esmvaltool is not available for esmvalcore testing (E ModuleNotFoundError: No module named 'esmvaltool'). This would make sense, because you want to be able to test esmvalcore standalone without a dependency on esmvaltool.

The easiest solution would be to just add it here instead of in this repository: https://github.com/ESMValGroup/ESMValTool/blob/b8a7b364d141c404a49cae33e7aeabb5fbf7cc36/setup.py#L255

Or are you keen on including this in ESMValCore?

[alistairsellar]

The easiest solution would be to just add it here instead of in this repository: https://github.com/ESMValGroup/ESMValTool/blob/b8a7b364d141c404a49cae33e7aeabb5fbf7cc36/setup.py#L255

Ahah! Thanks @bouweandela - I'll do that.