Add hierarchy scheme iterators to `Topology`

[mattwthompson]

Resolves #1601

Possibly blocked by #1660, would be convenient for a path forward with #1659 to also be decided.

• [x] Tag issue being addressed
• [ ] Add tests
  • [x] Error if any molecule is misses requested iterator
  • [x] Order hierarchy elements by molecule, then whatever sorting is defined by the scheme itself
  • [x] Make no effort to sort between molecules
  • [ ] Error out if a name conflict with existing attributes is found
• [x] Update docstrings/documentation, if applicable
• [x] Lint codebase
• [x] Update changelog

[codecov[bot]]

Codecov Report

Merging #1661 (faab121) into main (615320f) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files