Add SlicerIGSpineDeformity extension

[SenonETS]

Distribute the custom extension SlicerIGSpineDeformity

New extension

• [x] Extension has a reasonable name (not too general, not too narrow, suggests what the extension is for)
• [x] Repository name is Slicer+ExtensionName
• [x] 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
• [x] Extension description summarizes in 1-2 sentences what the extension is usable (should be understandable for non-experts)
• [x] Any known related patents must be mentioned in the extension description.
• [x] If source code license is more restrictive for users than BSD, MIT, Apache, or 3D Slicer license then the name of the used license must be mentioned in the extension description.
• [x] Extension URL and revision (scmurl, scmrevision) is correct, consider using a branch name (main, release, ...) instead of a specific git has to avoid re-submitting pull request whenever the extension is updated
• [x] 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)
• [x] Screenshot URLs (screenshoturls) are correct, contains at least one
• [x] Homepage URL points to valid webpage containing the following:
  • [x] Extension name
  • [x] Short description: 1-2 sentences, which summarizes what the extension is usable for
  • [x] At least one nice, informative image, that illustrates what the extension can do. It may be a screenshot.
  • [x] Description of contained modules: at one sentence for each module
  • [x] Tutorial: step-by-step description of at least the most typical use case, include a few screenshots, provide download links to sample input data set
  • [x] Publication: link to publication and/or to PubMed reference (if available)
  • [x] License: 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. We suggest you use the Slicer License or the Apache 2.0. Always mention in your README file the license you have chosen. If you choose a different license, explain why to the extension maintainers. Depending on the license we may not be able to host your work. Read here to learn more about licenses.
  • [x] Content of submitted s4ext file is consistent with the top-level CMakeLists.txt file in the repository (description, URLs, dependencies, etc. are the same)