Fixes MDAnalysis.analysis.density.convert_density() method has an invalid default unit value

[tanishy7777]

Fixes #4829

Changes made in this Pull Request:

• Changed the default unit in the convert_density() function to Angstrom^{-3} from Angstrom
• Added new tests to check if the changes to the code(default unit) solve the issue in the testsuite/MDAnalysisTests/analysis/test_density.py

Before

def convert_density(self, unit='Angstrom'):

After

def convert_density(self, unit='Angstrom^{-3}'):

PR Checklist

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

Developers certificate of origin

• I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

---

📚 Documentation preview 📚: https://mdanalysis--4833.org.readthedocs.build/en/4833/

[tanishy7777]

Before the changes the test was failing as shown below

After making the changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.63%. Comparing base (fa73478) to head (69a6d83).
Report is 46 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4833      +/-   ##
===========================================
- Coverage    93.65%   93.63%   -0.03%     
===========================================
  Files          177      189      +12     
  Lines        21763    22829    +1066     
  Branches      3065     3065              
===========================================
+ Hits         20382    21375     +993     
- Misses         929     1002      +73     
  Partials       452      452              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[RMeli]

Thanks @tanishy7777, LGTM!

[RMeli]

Conflicts needs to be resolved here too, otherwise it's good to go.