KVTree release for SCR v3.1

[CamStan]

See SCR v4.0 project: https://github.com/orgs/LLNL/projects/26

Process

• Merge any outstanding PRs, close any outstanding issues
• Pass GitLab CI tests
• Update the version number in the code, CMakeLists.txt, and any references to dependency versions
• Create a PR with these changes
• Merge the PR
• Tag a release on GitHub
• write some release notes
• Update the spack package: Update hash for download; Update dependency versions