Fix for overwriting isocenter with NaN from the GUI when multiple isocenters are defined

[wahln]

Bug report #867 highlighted that, when pln.propStf.isoCenter contains multiple, differing isocenters, updates in the PlanWidget will parse the textbox value ("multiple isoCenters") incorrectly and assign NaN to pln.propStf.isoCenter.

This PR introduces:

• A test to capture and reproduce this scenario
• A check for the string value in case of multiple isocenters, avoiding overwriting of the field
• An additional cleanup restoring octave compatibility of PlanWidget field changes

Fixes #867

[Copilot]

Pull Request Overview

This PR fixes a bug where the GUI would overwrite multiple isocenters with NaN values when updating plan parameters. The issue occurred when the PlanWidget tried to parse the "multiple isoCenter" string as a numeric value.

• Adds test coverage for the multiple isocenter scenario
• Implements a string check to prevent overwriting when "multiple isoCenter" is displayed
• Improves Octave compatibility by storing object tag in a variable

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
test/gui/test_gui_PlanWidget.m	Adds comprehensive test to verify multiple isocenter handling during GUI updates
matRad/gui/widgets/matRad_PlanWidget.m	Implements fix to preserve multiple isocenters and improves Octave compatibility

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.17%. Comparing base (c67b2a9) to head (cbb274c).
⚠️ Report is 5 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #868      +/-   ##
==========================================
+ Coverage   53.07%   53.17%   +0.10%     
==========================================
  Files         307      307              
  Lines       19819    19822       +3     
==========================================
+ Hits        10519    10541      +22     
+ Misses       9300     9281      -19     

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[github-actions]

Test Results

  3 files  ±0    3 suites  ±0   42m 42s ⏱️ +19s
289 tests +1  289 ✅ +1  0 💤 ±0  0 ❌ ±0 
963 runs  +3  958 ✅ +3  5 💤 ±0  0 ❌ ±0 

Results for commit cbb274c. ± Comparison against base commit c67b2a9.

♻️ This comment has been updated with latest results.

[github-actions]

Package	Line Rate	Health
coverage Package 1	53%	➖
Summary	53% (10534 / 19822)	➖