[WIP] Feature full multigrid

[bigfooted]

Proposed Changes

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Fixes some small issues preventing efficient use of full multigrid
Fixes a small issue with the implicit line agglomeration - for single core switching on implicit line agglomeration for a uniform mesh with all squares will now lead to a coarse mesh with uniform squares. with inflation layers you can choose to agglomerate anisotropically (in the flow direction only) or isotropically.

TODO

• FMG testcase

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.

PR Checklist

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