Eikehmueller/gtmgpc restriction

[eikehmueller]

Description

Add functionality to pass an optional restriction matrix to GTMGPC.

fixes #4367

At the moment, only the interpolation matrix of the pcmg object can be specified through the appctx via "interpolation_matrix":interp_petscmat and the restriction matrix is automatically set to the transpose of this. The changes in this PR allow passing the restriction matrix through the appctx via "restriction_matrix":restr_petscmat. If no restriction matrix is specified, it defaults to the transpose of the interpolation matrix as before, hence the change is backward compatible.

[pbrubeck]

Could you checkout release, pull, and merge release into this branch so the unrelated commits go away?

[eikehmueller]

just did

git checkout release
git pull
git checkout eikehmueller/gtmgpc-restriction
git merge release
git push

but that doesn't seem to change anything.

[pbrubeck]

Ah, the merge was set into master, I just changed it now to release

[pbrubeck]

This looks good but we need to add tests to make sure these features remain supported, including a test for the custom interpolation

[pbrubeck]

This looks good. Thanks!

[connorjward]

That exceeded expectations! Thank you!

[JHopeCollins]

LGTM

[JHopeCollins]

Once release is merged into master (hopefully later today) you can update this branch with the link fixes so all the workflows pass, and we can get this merged