Fix swarm block neighbor indexing in 1D, 2D

[brryan]

PR Summary

For some cases, the current block of particles was being computed incorrectly. This was because the swarm_comms.cpp initialization machinery would overwrite the region of neighbor_indices_h belonging to the currently active meshblock. I fixed this, and also changed the index we use to compute neighbor indices for unused dimensions in SwarmDeviceContext to improve consistency.

This should fix the downstream issue @pdmullen.

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]

Pretty trivial. LGTM. Is there a unit test we could add to catch this?

[brryan]

Pretty trivial. LGTM. Is there a unit test we could add to catch this?

I don't think this is easy to do as a unit test because ultimately we need to call Mesh::Initialize() to set up the data structures used by the swarm to compute neighbor blocks, but I added a check to the particles example that makes sure initialized particles report that they are on the current meshblock.