TVB-2855: Fixed Tracts Importers and Visualizer

Created by: VinczeRobert

I seemingly managed to fix the importers and the visualizer partially. One of the first problems I encountered is that judging by how the code is written I am pretty sure that the Region Volume Mapping when importing a Tracts should be a required, not an optional field. Paula sent me a RVM and a connectivity which I was able to import with the 5M.trk file and I also changed the code until I was able to see some tracts in the viewer. The problem is that it strongly seems that not only are the vertices not read in slices, but there are actually more vertices read than they are said to be in the H5 which clearly means that the reading of vertices in slices is what still needs to be fixed.