ENH: add structural to total mass ratio for motor and rocket

[Lucas-Prates]

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

New behavior

Solves issue #671 .

Breaking change

• [x] No

Additional information

From what I understood from the issue, the information required to be printed was the the ratio of the dry mass by the total mass. I saw some sources computing the propellant mass ratio, which I believe is the opposite (inverse of the specified ratio).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.96%. Comparing base (13e66f6) to head (e532489). Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
rocketpy/motors/motor.py	75.00%	2 Missing :warning:
rocketpy/rocket/rocket.py	75.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #713      +/-   ##
===========================================
+ Coverage    75.88%   75.96%   +0.07%     
===========================================
  Files           99       99              
  Lines        11222    11258      +36     
===========================================
+ Hits          8516     8552      +36     
  Misses        2706     2706              

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[Lucas-Prates]

Made the suggested changes. The PR is ready for review again!