ENH: Adds GenericMotor.load_from_eng_file() method

[juliomachad0]

Pull request type

• [x] Code changes (bugfix, features)

Checklist

• [x] Lint (black rocketpy/ tests/) has passed locally
• [x] All tests (pytest tests -m slow --runslow) have passed locally

Current behavior

the static method to create motors from eng files did not exist.

New behavior

method created and working

Breaking change

• [x] No

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.

Project coverage is 74.47%. Comparing base (3a4c742) to head (7e33586). Report is 43 commits behind head on develop.

Files with missing lines	Patch %	Lines
rocketpy/motors/motor.py	89.18%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #676 +/- ## =========================================== - Coverage 75.87% 74.47% -1.41% =========================================== Files 85 89 +4 Lines 10085 10369 +284 =========================================== + Hits 7652 7722 +70 - Misses 2433 2647 +214 ```

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