Small: Changes to DataCollection::Add and MeshBlockData::Intialize

[lroberts36]

PR Summary

This PR does two related things:

1. It adds a check in DataCollection::Add that if a container already exists, it was created with the exact same list of fields as were passed into the current call to Add. Previously, it was just checked that the pre-existing container had a superset of the requested fields, which lead to a communication bug in multigrid that took me a long time to figure out.
2. Adds the ability to call DataCollection::Add on a partition with a list of fields to include. Previously, this resulted in a PARTHENON_FAIL.

I am open to arguments on 1) why this isn't the behavior we desire.

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]

LGTM