Add nodegroup support to control system and interpolation framework

[nilsdeppe]

Proposed changes

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Further comments

[knelli2]

I had added comments for the previous commits asking to do the opposite of this. Could you give a brief explanation? Was this necessary to compile?

[nilsdeppe]

We've generally found "reaching up" into the metavariables to be fairly brittle and tricky. Considering we deduce the dim pretty much everywhere in the code I saw no reason to avoid doing that here. I've also sometimes had issues with GCC not being able to retrieve Metavaraibles::volume_dim and those were extremely difficult to debug, so I feel like suggesting that as the "default" is not really a good thing.

[knelli2]

I agree in general we shouldn't reach up, but I thought the volume_dim was an exception since it's so ubiquitous in our metavars. We also do this a lot throughout the code for the dim, so we aren't really consistent. But if it's causing issues with GCC then passing in the Dim explicitly is ok

[nilsdeppe]

@knelli2 I pushed fixups. Thanks for the review :)

[knelli2]

Looks good. You can squash

[nilsdeppe]

Rebased and squashed. Thanks for the reviews!