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).