Moving plugins out in external sofa-framework repositories (STC#9)

[hugtalbot]

As decided at the last STC, a serie of plugins will be moved out from the SOFA repository and saved into their own repository. List of related plugins available here.

This will help in the process of deprecation and pluginization in SOFA.

• Extraction process is clearly explained in this documentation
• Remaining: making a pull-request in SOFA to
  • remove all plugin files
  • add a plugin folder containing one file ExternalProjectConfig.cmake.in
  • edit applications/plugin/CMakeLists.txt to replace sofa_add_plugin by sofa_add_plugin_external
• Finalize the GitHub repository:
  • add a description
  • add reviewers (sofa-framework/reviewers, sofa-framework/consortium-staff and appropriate watcher)
  • add the "plugin" topic (flag)

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[alxbilger]

@bakpaul @hugtalbot @olivier-roussel We discussed last week that SofaNewMat and probably SofaDistanceGrid must be externalized, according to https://github.com/sofa-framework/sofa/issues/4221