ENH: change of variables between rocket and motor

[ompro07]

The reference point for the calculation of the center of mass of the motor was altered from the geometric center of the propellant grain to the nozzle

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• Code base additions (bugfix, features)
• Code maintenance (refactoring, formatting, renaming, tests)
• ReadMe, Docs and GitHub maintenance
• Other (please describe):

Pull request checklist

Please check if your PR fulfills the following requirements, depending on the type of PR:

• ReadMe, Docs and GitHub maintenance:

  • Spelling has been verified
  • Code docs are working correctly

• Code base maintenance (refactoring, formatting, renaming):

  • Docs have been reviewed and added / updated if needed
  • Lint (black rocketpy) has passed locally and any fixes were made
  • All tests (pytest --runslow) have passed locally

• Code base additions (for bug fixes / features):

  • Tests for the changes have been added
  • Docs have been reviewed and added / updated if needed
  • Lint (black rocketpy) has passed locally and any fixes were made
  • All tests (pytest --runslow) have passed locally

What is the current behavior?

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What is the new behavior?

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Does this introduce a breaking change?

• Yes
• No

Other information

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The reference point for the calculation of the center of mass of the motor was altered from the geometric center of the propellant grain to the nozzle, this will disable the notebooks

[Gui-FernandesBR]

It's indeed a good addition, but as we are requesting a breaking change I believe we should work a little bit more in some files.

Besides to above commentaries I left, I believe we also should adjust the following files to ensure they work well after approving and merging this PR, the adjustments should be short and easy to be done:

• docs\notebooks\getting_started.ipynb
• docs\notebooks\getting_started_colab.ipynb
• docs\notebooks\solid_motor_class_usage.ipynb
• opt*: docs\notebooks\dispersion_analysis\dispersion_analysis.ipynb

[Gui-FernandesBR]

HELP,
some of the tests are not working, however I could not find why ,
we need all checks running again before merging

[Gui-FernandesBR]

Everything is fine for me!
Thank you for taking care of every detail on the changed files so we guarantee the code additions are doing great.

And btw congrats for the smart and significant development, looking forward to seeing next PR from you @ompro07 !