[Trivial] add GetSize and GetIndex to sparse pack

[Yurlungur]

PR Summary

Upstreaming a few features we use in riot. Mainly a cheap way of getting the number of sparse IDs of a variable on a block and the "index" in the pack on a given block.

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

[pdmullen]

LGTM! if you are feeling brave, these can also be added to SwarmPacks

[lroberts36]

LGTM

[Yurlungur]

LGTM! if you are feeling brave, these can also be added to SwarmPacks

Can you click approve? Also are these relevant to SwarmPacks? I guess GetIndex is but since swarms don't have sparse IDs I don't think GetSize provides much value.

[pdmullen]

LGTM! if you are feeling brave, these can also be added to SwarmPacks

Can you click approve? Also are these relevant to SwarmPacks? I guess GetIndex is but since swarms don't have sparse IDs I don't think GetSize provides much value.

The member functions are more or less consistent between SparsePacks and SwarmPacks, so it might seem natural to add them, however, as you add, I also don't see an immediate need for them...