Add helper to create ring VOIs

[acsevillam]

Feature description

This PR adds a small geometry helper to create ring VOIs from existing structures. The helper builds a ring as an outer margin minus an inner margin around a base VOI, then clips the result to a limiting VOI.

The goal is to provide a reusable utility for structure preparation workflows without coupling it to a specific patient, script, or optimization strategy.

Approach

• Adds matRad_createRing under matRad/geometry.
• Supports all CT scenarios available in ct.numOfCtScen.
• Uses existing matRad_addMargin behavior for margin expansion.
• Returns the updated cst and the row index of the created ring VOI.
• Adds MOxUnit coverage for:
  • creating an outer-minus-inner ring,
  • excluding the base VOI from the ring,
  • clipping the ring to a limiting VOI.

No new global defaults are introduced, and no external dependencies or toolboxes are required.

Open Questions and/or Concerns

• CI should confirm the new geometry test on the supported MATLAB/Octave environments.

References

[read-the-docs-community]

Documentation build overview

📚 matRad | 🛠️ Build #32912336 | 📁 Comparing 07972af against latest (5a43850)

  🔍 Preview build  

19 files changed · + 1 added · ± 18 modified

+ Added

• datastructures/userfolder.html

± Modified

• genindex.html
• index.html
• api/dicom.html
• api/geometry.html
• api/index.html
• guide/gdosecalc.html
• guide/index.html
• overview/about.html
• overview/cite.html
• overview/faq.html
• and 8 more...

[codecov]

Codecov Report

❌ Patch coverage is 78.12500% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.00%. Comparing base (174e756) to head (07972af).

Files with missing lines	Patch %	Lines
matRad/geometry/matRad_createRing.m	78.12%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #917      +/-   ##
==========================================
+ Coverage   54.96%   55.00%   +0.03%     
==========================================
  Files         322      323       +1     
  Lines       20618    20650      +32     
==========================================
+ Hits        11333    11358      +25     
- Misses       9285     9292       +7     

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[github-actions]

Test Results

    3 files  ±0      3 suites  ±0   32m 17s ⏱️ +5s
  369 tests +2    369 ✅ +2  0 💤 ±0  0 ❌ ±0 
1 203 runs  +6  1 198 ✅ +6  5 💤 ±0  0 ❌ ±0 

Results for commit 07972af. ± Comparison against base commit 174e756.