WIP: Generalize multigrid hierarchy

[lroberts36]

PR Summary

• Generalizes the geometric multigrid (GMG) hierarchy to support an initial
  sequence of leaf-only GMG grids before transitioning to two-level
  composite grids, rather than forcing an immediate move to composite grids.

• Introduces a new runtime parameter parthenon/mesh/base_block_coarsenings
  which controls how many leaf GMG grids (with increasing in-block coarsening) are
  included at the start of the hierarchy.

• Refactors GMG grid identification to explicitly track:

  • grid type (leaf vs two_level_composite),
  • in-block coarsening level (block_coarsenings),
  • multigrid level index,
  • and whether a grid participates in the GMG hierarchy.

• Preserves and clarifies two-level composite grid semantics, where a composite
  grid at logical level ℓ includes:

  • level ℓ blocks (leaf + internal parents of finer leaves), and
  • level ℓ−1 leaf blocks to provide boundary conditions.

• Updates GMG hierarchy construction logic to cleanly separate:

  • leaf-only GMG stages,
  • tree traversal using two-level composite grids,
  • and subsequent in-block coarsening at the root.

• Extends boundary communication infrastructure to allow multiple
  communication buffers per communication channel, enabling concurrent
  communication on multiple MeshData stages without MPI lock contention.

  • Implements the above communication changes across:
    tag_map.*, bvals_utils.hpp, mesh_data.hpp, and build_boundary_buffers.cpp.

---

TODO before removing WIP

• Update coalesced communication infrastructure to correctly handle the new
  “multiple buffers per communication channel” capability.

• Add a more extensive solver testing infrastructure for GMG-based solvers.

• Add or update documentation describing:

  • the parthenon/mesh/base_block_coarsenings parameter,
  • the generalized GMG hierarchy structure,
  • and the updated boundary communication buffering model.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• 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.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[Yurlungur]

since this depends on all those other MRs... the diff is hard to read. I think I'll wait to review until those are in

[lroberts36]

since this depends on all those other MRs... the diff is hard to read. I think I'll wait to review until those are in

@Yurlungur: Yeah, no need to review while it is in WIP. I am still working on it and I put it up mostly because I wanted to easily be able to see the diff.

[lroberts36]

This was just a previously existing bug where the NDIM being passed here was assumed to be the multigrid level.