Add iris-esmf-regrid as a dependency

[zklaus]

Description

This simply adds iris-esmf-regrid as a dependency, making it easily available for regridding. We do not, so far, use it in any integrated recipes yet.

• Closes #issue_number

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[valeriupredoi]

cheers @zklaus 🍺

[bouweandela]

@zklaus Wouldn't it make more sense to add iris-esmf-regrid as a dependency to esmvalcore instead?

[valeriupredoi]

@bouweandela fairly sure Klaus will, but I hurried him up a bit so I can have it plopped in the release; same as with iris that is in both Core and Tool requirements - it'll be used in diags too

[zklaus]

Actually, I thought a little bit about it and decided against that. The reason is that it is really not used in the core, but just something made available for recipe developers. I didn't think too long though and have not developed strong feelings about it, so it's open for discussion.

[bouweandela]

Let's talk about it at the next tech lead team meeting then: ESMValGroup/Community#56 (comment).