Add kwarg n_frames to Universe.empty() (#4110)

[xhgchen]

• Resolves Add kwarg to make a Universe with multiple frames from scratch #4110
• Add kwarg n_frames to class method empty() in MDAnalysis.core.universe
• Add unit test to verify that n_frames creates an empty Universe with the specified number of frames

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

---

📚 Documentation preview 📚: https://mdanalysis--4140.org.readthedocs.build/en/4140/

[github-actions]

Linter Bot Results:

Hi @xhgchen! Thanks for making this PR. We linted your code and found the following:

Some issues were found with the formatting of your code.

Code Location	Outcome
main package	⚠️ Possible failure
testsuite	✅ Passed

Please have a look at the darker-main-code and darker-test-code steps here for more details: https://github.com/MDAnalysis/mdanalysis/actions/runs/4998399631/jobs/8953767483

---

Please note: The black linter is purely informational, you can safely ignore these outcomes if there are no flake8 failures!

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e46c5ef) 93.59% compared to head (11c43b7) 93.59%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4140   +/-   ##
========================================
  Coverage    93.59%   93.59%           
========================================
  Files          192      192           
  Lines        25143    25143           
  Branches      4058     4058           
========================================
  Hits         23532    23532           
  Misses        1093     1093           
  Partials       518      518           

Impacted Files	Coverage Δ
package/MDAnalysis/core/universe.py	97.26% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.