FIX: Test problem in ComputeTubeProbability due to wrong MIDAS data.

     This should fix the failing ComputeTubeProbability test. The
     TubeTK application ComputeTubeProbability expects an input
     image of type short while the test image on MIDAS was of type
     float which caused all kinds of weird behaviour over different
     platforms.

     Baselines should be fixed with this commit.