Properly free swarm boundary MPI requests

[brryan]

PR Summary

@pdmullen identified a bug in a downstream application using Parthenon particles where after a number of timesteps, MPI would fail internally. It turns out that Parthenon was not properly freeing MPI requests for swarms, but just setting its own variable to null. This is an easy fix, and solved the downstream issue.

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

[lroberts36]

LGTM

[brryan]

A simple but important fix; setting to auto-merge