Add host labels and an overload for operator<< in sparsepack

[Yurlungur]

PR Summary

I'm chasing down a bug in riot right now and it's been useful to be able to print the contents of a SparsePack object. This adds the capability to do so with a LabelHost function that can be called on host and a stream operator overload if you just want to dump the whole pack.

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, seems very helpful

[pgrete]

LGTM