TST: Add unit test for get_value_opt method

[Gui-FernandesBR]

Pull request type

• Other (please describe): Tests

Checklist

• Tests for the changes have been added (if needed)
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest --runslow) have passed locally

Current behavior

The tests don't cover some lines of the get_value_opt when the interpolation method is set to shepard.

New behavior

Added this new test function, let's see.

Breaking change

• No

Additional information

Trying to support the #501

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (enh/get-value-opt-method@d4dbbce). Click here to learn what that means.

Additional details and impacted files

@@                     Coverage Diff                     @@
##             enh/get-value-opt-method     #505   +/-   ##
===========================================================
  Coverage                            ?   71.16%           
===========================================================
  Files                               ?       55           
  Lines                               ?     9273           
  Branches                            ?        0           
===========================================================
  Hits                                ?     6599           
  Misses                              ?     2674           
  Partials                            ?        0           

Flag	Coverage Δ
unittests	71.16% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.