Add GetPackDimension to StateDescriptor

[bprather]

Sometimes when initializing a package, you want to know how many variables another package has declared (e.g., a Flux package might need the total number of WithFluxes variables declared so far). The Mesh doesn't exist yet, so you have to ask the other packages directly, and have them tally up variable lists. Thus, GetPackDimension.

This is the last of the patches I've been carrying in KHARMA's version of Parthenon vs. develop. It works for my purposes downstream, but I'm pretty sure it would fail for Overridable variables and maybe other corner cases. Happy to try to make it more robust if it seems useful, or keep it downstream if no one else needs this kind of info at package init time (or maybe there's some cleaner way to get it? Create a temporary resolved_state or something?)

I've checked and these functions work fine for the StateDescriptor returned by CreateResolvedStateDescriptor, which I think already resolves any Overridable variables down to a single entry each. So, if anyone else needs this sort of crazy thing, the code in here at least has a decent chance of being robust for whatever use case they're throwing at it. Since I think the overall function has a chance of being broadly useful, I'm going to remove WIP, add a changelog entry, and ask for reviews/opinions.

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

[Yurlungur]

Minor comments below.