Thread UIDs through `data_collection`

[Yurlungur]

PR Summary

This PR adds the ability to request a new MeshData or MeshBlockData object by requesting a subset of unique IDs of variables from the parent object, rather than by using only variable names. This should make UID-based operations more usable and thus unlock a bit of performance by minimizing string comparisons. Needed in riot and I suspect also KHARMA.

PR Checklist

• [x] Code passes cpplint
• [ ] New features are documented.
• [x] Adds a test for any bugs fixed. Adds tests for new features.
• [x] Code is formatted
• [x] Changes are summarized in CHANGELOG.md
• [ ] Change is breaking (API, behavior, ...)
  • [ ] Change is additionally added to CHANGELOG.md in the breaking section
  • [ ] PR is marked as breaking
  • [ ] Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• [ ] CI has been triggered on Darwin for performance regression tests.
• [x] Docs build
• [x] (@lanl.gov employees) Update copyright on changed files

[Yurlungur]

Thanks for the review @pgrete I think I've addressed your comments now.

[Yurlungur]

@lroberts36 thanks for the review. I think I've addressed all your comments.

[pgrete]

With the latest changes the distinction between if (grid.type == GridType::two_level_composite) { and using pmy_mesh_->gmg_block_lists[src->grid.logical_level][i] is gone. Is that not required any more?

[lroberts36]

@pgrete: Yeah, that is no longer required since the src MeshData already should point to internal/multigrid blocks.