Add warnings for multiple molecules and tests

[j-wags]

• Fixes Enforce that Molecule cannot represent multiple molecules #1587
• Add tests
• Update docstrings/documentation, if applicable
• Lint codebase
• Update changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.76%. Comparing base (747c2f1) to head (053a24e).
Report is 1 commits behind head on main.

Additional details and impacted files

[j-wags]

Hm, CI currently indicates that this this leads to a consistent, significant slowdown that's tripping a timeout on an unrelated test. I don't want to merge with failing tests and it's possible that another approach wouldn't have this slowdown, so I'll kick CI now and come back to this Monday.

[j-wags]

Reran twice over the weekend and it came out all green, so I'll just merge this and I can relax the timeout later if needed.