ENH: Update IntensitySegmenter from r12 to r13

BUG: include(ExternalData) was missing.

Project was not configuring with CMake 2.8*, but only with CMake 3. Inclusion of CTest and ExternalData has been moved to main CMakeLists (not superbuild) to be able to run the tests from the main folder.

http://www.nitrc.org/plugins/scmsvn/viewcvs.php?view=rev&root=dentaltools&revision=13
Merge pull request #797 from fbudin69500/update-IntensitySegmenter

ENH: Update IntensitySegmenter from r12 to r13