Esmvaltool: Add often used scientific regions to ESMValTool

[lukasbrunner]

Frequently used scientific regions such as the SREX regions Seneviratne et al. (2012) or the extended AR6 regions Iturbide et al. (2020) could be added to the ESMValTool/Core for convenience.

I currently need several of these regions and there is a way of cutting them already following the example recipe here:
https://docs.esmvaltool.org/en/latest/recipes/recipe_shapeselect.html

However in the long term I could imagine it would be nice to have them readily available. I see at least two options how this could work:

• upload the shapefiles to the Tool so that they can be used
• the regionmask package (https://regionmask.readthedocs.io/en/stable/index.html) by @mathause et al. already allows convenient selection of scientific regions as well as countries.

I am certainly happy to help implement this but I guess I would also need some help. But please do let me know you would consider having these regions available by default helpful at all first.

[stefsmeets]

There is a related PR here: https://github.com/ESMValGroup/ESMValCore/issues/758

[lukasbrunner]

Ah good point I had not checked in the Core! But I guess it rather belongs there anyway. I just started with Peters example, therefore I ended up here...

[valeriupredoi]

I believe this is a Core issue, does this need to be in a diagnostic? The codebase will live in Core, and might actually be an add-on in ESMValGroup/ESMValCore#758 as @stefsmeets points out

[Peter9192]

Shall we close this issue and continue the discussion in https://github.com/ESMValGroup/ESMValCore/issues/758? @lukasbrunner