Add Contains to Sparse Pack

[Yurlungur]

PR Summary

Syntactic sugar for

bool var_v_is_in_pack = (pack.GetUpperBoundHost(0, v()) >= 0);

This was suggested by @brryan in a downstream PR in Phoebus. I think it helps clarify the cases when we pack on many variables, of which only some are present.

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

[brryan]

One question but LGTM and I'll approve now.

[lroberts36]

This is a nice feature

[lroberts36]

LGTM