Fix save and load methods for coupling element

[jguarato]

Fixes #1135

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Note:

• Some tests were included for the CouplingElement in test_shaft_element.py
• The use of the CouplingElement was also included in the ShaftElement class part of the ROSS tutorial.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.75%. Comparing base (f1bba84) to head (dac2556).
Report is 66 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1141      +/-   ##
==========================================
+ Coverage   81.67%   82.75%   +1.07%     
==========================================
  Files          38       38              
  Lines        8310     8012     -298     
==========================================
- Hits         6787     6630     -157     
+ Misses       1523     1382     -141     

Files with missing lines	Coverage Δ
ross/coupling_element.py	84.29% <100.00%> (+70.35%)	⬆️

... and 2 files with indirect coverage changes

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