ENH: Update Superbuild process:

-External tools source code is downloaded in ShapePopulationViewer directory by default
- Compiles with VTK5 and VTK6
ENH : Update CMakeLists.txt to be able to commit source code for the new version of the Slicer Extension
ENH : Change extension category to SPHARM-PDM category
ENH: Use USE_VTKv6 instead of USE_VTK_6 to match what is done in Slicer