Alter the code to remove the assumption that the first part of descendants is the root

[raduiordache]

Refactored several retrieval tests to split long lines into multiple lines, improving readability. Also adjusted the expected child instances count. Furthermore, added an entry to the CHANGELOG regarding a refactor about retrieving descendants of a 'Part'.

Fixes #

Checklist:

• [x] My code follows the pykechain style
  • [x] I added type hinting to all functions
  • [x] I added documentation for all public functions
  • [x] I locally used tox to check for dists and docs errors
  • [x] My code passes the pep8 and flake8 linting checks and no warnings
  • [x] I removed unused imports, using Code > Optimize Imports in PyCharm
  • [x] I used black to format the new code
• [x] I have added tests that prove my fix is effective or that my feature works
  • [x] I committed fresh test cassettes
  • [x] I have proper test coverage (don't decline the coverage of the test)
• [x] I asked another teammate to review the code
• [x] I update the Changelog.md with the appropriate changes.