Small Bugfix: Remove shared_ptr cycle in Tree

[lroberts36]

PR Summary

This PR removes a memory leak caused by a closed shared_ptr cycle in forest::Tree by switching the neighbor pointers from shared_ptr to non-owning raw pointers. This probably had limited effect since the forest is only destroyed at the very end of the process. I found this using -DENABLE_ASAN=On.

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