FIX: Still failing ComputeTubeProbability test.

     This commit SHOULD fix the still failing ComputeTubeProbability
     test.

     The previous commits to fix the failing test were related
     to a wrong voxel type in the density image.

     Apparently, it is also a problem to consistently load .tre
     files (with very dense tube point sampling) from ASCII files
     across different platforms. The test was failing since tube
     points were mapped to voxel indices in an inconsistent manner
     across platforms (e.g., Mac vs. Linux).
Merge pull request #117 from rkwitt/Fix-ComputeTubeProbability-FinalShot

FIX: Still failing ComputeTubeProbability test.
ENH: Speed-up TubesToDensityImage testing.

     Changed testing data and baselines to simple small images.
Merge pull request #118 from rkwitt/Enh-TubesToDensityImage-Runtime

ENH: Speed-up TubesToDensityImage testing.
ENH: Update CTestCustom.cmake.in
ENH: Create TubeTKValgrindSuppressions.txt
ENH: Update CTestCustom.cmake.in
ENH: Sync ITK version with 3D Slicer
Merge pull request #119 from jamiesnape/SyncWithSlicerYetAgain

ENH: Sync ITK version with 3D Slicer
ENH: Update CTestCustom.cmake.in
ENH: Enable use of upstream LIBSVM source
ENH: Use stable source tar ball for Cppcheck
Merge pull request #123 from jamiesnape/UseUpstreamLIBSVM

ENH: Enable use of upstream LIBSVM source
ENH: Update CTestCustom.cmake.in
ENH: Use stable tar ball for ITK and other minor superbuild fixes
Merge pull request #124 from jamiesnape/UseStableITKTarball

ENH: Use stable tar ball for ITK and other minor superbuild fixes
BUG: Update itktubeImageToTubeRigidMetric.hxx

Closes #125.
STYLE: Update CMakeLists.txt
BUG: Fix superbuild download issues
ENH: Try different URL for ITK source tarball