SVN moved over to opensg.vrsource.org

The OpenSG SVN has been moved to https://opensg.vrsource.org/svn.

This move allows us to integrate it directly into Trac and put links to files in the repository as well as to specific revisions in the Wiki. It will simplify communication and make things more efficient. From now on please use Trac tickets for all your source-related communication like bugs, feature requests etc.