DVH Widget Bugfix

[wahln]

This pull request includes several changes to improve the functionality and testing of the matRad GUI widgets. The most important changes involve enhancing the matRad_DVHWidget and its showDVH function, updating the matRad_showDVH function, and adding new test cases for various widgets.

Enhancements to showDVH function:

• matRad/gui/widgets/matRad_DVHWidget.m: Added logic to handle scenarios more robustly by introducing the ct variable and updating the scenario model creation and checks.

Updates to matRad_showDVH function:

• matRad/planAnalysis/matRad_showDVH.m: Modified the xlabel function to include the axesHandle parameter for better plot labeling.

Addition of new test cases:

• test/gui/test_gui_DVHStatsWidget.m: Added multiple test functions to construct DVHStatsWidget with different plan types (Photon, Proton, Carbon, Helium) and verify their correct instantiation.
• test/gui/test_gui_PlanWidget.m: Added test functions to construct PlanWidget with different plan types (TG119, Photon, Proton, Carbon, Helium) and verify their correct instantiation. [1] [2]
• test/gui/test_gui_exportWidget.m: Added a test function to construct exportWidget with a Carbon plan and verify its correct instantiation.
• test/gui/test_gui_viewingWidget.m: Added test functions to construct ViewingWidget with different plan types (TG119, Photon, Proton, Carbon, Helium) and verify their correct instantiation. [1] [2]

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.26%. Comparing base (c55fa64) to head (3f50eb7). Report is 3 commits behind head on dev.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
matRad/gui/widgets/matRad_DVHWidget.m	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #793 +/- ## ========================================== + Coverage 50.27% 51.26% +0.99% ========================================== Files 598 598 Lines 37262 37272 +10 ========================================== + Hits 18732 19108 +376 + Misses 18530 18164 -366 ```

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[github-actions[bot]]

Package	Line Rate	Health
coverage Package 1	51%	➖
Summary	51% (9547 / 18636)	➖

[github-actions[bot]]

Test Results

  3 files  ± 0    3 suites  ±0   33m 57s ⏱️ +25s 274 tests +13  274 ✅ +13  0 💤 ±0  0 ❌ ±0  918 runs  +39  913 ✅ +39  5 💤 ±0  0 ❌ ±0 

Results for commit 3f50eb7b. ± Comparison against base commit c55fa648.

:recycle: This comment has been updated with latest results.