New extension

This PR adds MRGelDosimetry, which is a 3D Slicer extension for MR-based gel dosimetry analysis. Building on the existing optical CT gel dosimetry workflow, this extension adapts the pipeline for MR imaging and provides a streamlined workflow for ΔR1 map generation, image registration, dose calibration, and dose comparison.

Tier 1

• Extension has a reasonable name (not too general, not too narrow, suggests what the extension is for). The extension name should not start with Slicer (unless it explicitly provides a bridge between Slicer and a tool or library), because it would make it more difficult to find extensions if the name of many started with the same word.
• Repository name is Slicer+ExtensionName (except if the repository that hosts the extension can be also used without Slicer)
• Repository is associated with 3d-slicer-extension GitHub topic so that it is listed here. To edit topics, click the settings icon in the right side of "About" section header and enter 3d-slicer-extension in "Topics" and click "Save changes". To learn more about topics, read https://help.github.com/en/articles/about-topics
• Extension description summarizes in 1-2 sentences what the extension is usable (should be understandable for non-experts)
• Any known related patents must be mentioned in the extension description.
• LICENSE.txt is present in the repository root and the name of the license is mentioned in extension homepage. We suggest you use a permissive license that includes patent and contribution clauses. This will help protect developers and ensure the code remains freely available. MIT (https://choosealicense.com/licenses/mit/) or Apache (https://choosealicense.com/licenses/apache-2.0/) license is recommended. Read here to learn more about licenses. If source code license is more restrictive for users than MIT, BSD, Apache, or 3D Slicer license then describe the reason for the license choice and include the name of the used license in the extension description.
• Extension URL and revision (scmurl, scmrevision) is correct, consider using a branch name (main, release, ...) instead of a specific git hash to avoid re-submitting pull request whenever the extension is updated
• Extension icon URL is correct (do not use the icon's webpage but the raw data download URL that you get from the download button - it should look something like this: https://raw.githubusercontent.com/user/repo/main/SomeIcon.png)
• Screenshot URLs (screenshoturls) are correct, contains at least one
• Content of submitted json file is consistent with the top-level CMakeLists.txt file in the repository (dependencies, etc. are the same)
• Homepage URL points to valid webpage containing the following:
  • Extension name
  • Short description: 1-2 sentences, which summarizes what the extension is usable for
  • At least one nice, informative image, that illustrates what the extension can do. It may be a screenshot.
  • Description of contained modules: at one sentence for each module
  • Publication: link to publication and/or to PubMed reference (if available)
• Hide unused github features (such as Wiki, Projects, and Discussions, Releases, Packages) in the repository to reduce noise/irrelevant information:
  • Click Settings and in repository settings uncheck Wiki, Projects, and Discussions (if they are currently not used).
  • Click the settings icon next to About in the top-right corner of the repository main page and uncheck Releases and Packages (if they are currently not used)
• The extension is safe:
  • Does not include or download binaries from unreliable sources
  • Does not send any information anywhere without user consent (explicit opt-in is required)

Tier 3

• Documentation, tutorial, and test data are provided for most modules. A tutorial provides step-by-step description of at least the most typical use case, include a few screenshots. Any sample data sets that is used in tutorials must be registered with the Sample Data module to provide easy access to the user.
• Follows programming and user interface conventions of 3D Slicer (e.g., GUI and logic are separated, usage of popups is minimized, no unnecessary custom GUI styling, etc.)
• The extension can be successfully built and packaged on all supported platforms (Windows, macOS, Linux)
• Maintainers respond to issues and pull request submitted to the extension's repository.
• Maintainers respond to questions directly addressed to him/her via @mention on the Slicer Forum.
• Permissive license is used for the main functions of the extension (recommended Apache or MIT). The extension can provide additional functionality in optional components that are distributed with non-permissive license, but the user has to explicitly approve those before using them (e.g., a pop-up can be displayed that explains the licensing terms and the user has to acknowledge them to proceed).

The checklist will be updated as more items are completed and additional components become ready for submission.