Hide Tract DataType until it is declared complete.

git-svn-id: https://repo.thevirtualbrain.org/svn/tvb/trunk/scientific_library@5965 1c0e02f0-7929-43c0-8fb3-3293bf43b0d1
Merge branch 'trunk' of https://github.com/the-virtual-brain/scientific_library into trunk