MNT: refactor u_dot parachute method

[Gui-FernandesBR]

Pull request type

• [x] Code maintenance (refactoring, formatting, tests)

Checklist

• [ ] Tests for the changes have been added (not needed)
• [ ] Docs have been reviewed and added / updated
• [x] Lint (black rocketpy/ tests/) has passed locally
• [x] All tests (pytest tests -m slow --runslow) have passed locally
• [ ] CHANGELOG.md has been updated (if relevant)

Current behavior

Let's be honest, reading this function isn't that easy.

New behavior

Changes names of variables and avoid some redundant operations.

Breaking change

• [x] No

Additional information

• This can help: https://downloads.hindawi.com/journals/ijae/2012/182907.pdf
• In the future, we should be adding new attributes to the parachute class (e.g. parachute radius, ka, etc.) and using them to improve the u_dot_parachute results.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.38%. Comparing base (61ebbc7) to head (db9a370). Report is 25 commits behind head on develop.

:exclamation: Current head db9a370 differs from pull request most recent head 64bdab2

Please upload reports for the commit 64bdab2 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #596      +/-   ##
===========================================
+ Coverage    73.37%   73.38%   +0.01%     
===========================================
  Files           57       57              
  Lines         9453     9461       +8     
===========================================
+ Hits          6936     6943       +7     
- Misses        2517     2518       +1     

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[Gui-FernandesBR]

@phmbressan could you re-approve it please?