Enable parallelization for analysis.polymer

[tanishy7777]

Fixes #4671

Changes made in this Pull Request:

• Enabled parallelization for PersistenceLength class
• added tests (including fixtures in analysis/conftest.py)
• update CHANGELOG

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--4871.org.readthedocs.build/en/4871/

[pep8speaks]

Hello @tanishy7777! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file package/MDAnalysis/analysis/polymer.py:

Line 308:80: E501 line too long (89 > 79 characters)

• In the file testsuite/MDAnalysisTests/analysis/conftest.py:

Line 172:1: E302 expected 2 blank lines, found 1
Line 174:25: W292 no newline at end of file

• In the file testsuite/MDAnalysisTests/analysis/test_persistencelength.py:

Line 174:1: E302 expected 2 blank lines, found 1