Updates to #1365

[lroberts36]

PR Summary

Original PR #1365

• Don't call boundary conditions on the coarse buffer if there are no coarser neighbors to fill it
• This is a correctness fix, not an optimization

Additions

1. Added HasCoarserNeighbors() method

   • Cleaner way to check if a block has coarser neighbors
   • Used in boundary_conditions.cpp and bnd_info.cpp where we were doing this check manually

2. Fixed uninitialized variables

   • Initialized all the uninitialized MeshBlock members:
     • gid, lid, cnghost, gflag
     • new_block_dt_, new_block_dt_hyperbolic_, new_block_dt_parabolic_
     • cost_, mem_usage_
   • Not really related to this PR, but good excuse to do it

3. Made neighbor vectors private

   • Neighbor lists now have public getters but can only be set through SetMeshBlockNeighbors()
   • Prevents accidental modification
   • Used vscode Continue + Claude-4.5 for this refactor, which was fairly efficient

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]

Nice clean up.

[Yurlungur]

Missing newline

[adamdempsey90]

WIP?

[Yurlungur]

Needs changelog and remove WIP