BUG: RidgeExtractor set xstep to 3-vector regardless of image dim
BUG: Use NRRD for the 6D ImageIO instead of MetaImage.

Try to avoid some 'double free or memory corruption' errors.
Merge pull request #321 from aylward/RidgeExtractorDimensionalityBug

BUG: RidgeExtractor set xstep to 3-vector regardless of image dim
Fix Convert TRE xml and readme
Improve spatial object node support

The vtkMRMLSpatialObjectNode used to only support VesselTubeSpatialObjects
which can be a problem as the files from Convert TRE can be only
TubeSpatialObjects (VesselTubeSpatialObject's superclass).
This commit allow the vtkMRMLSpatialObjectNode to support loading
TubeSpatialObject.
Also replace static_cast by dynamic_cast.
Merge pull request #322 from thewtex/nrrd6d

BUG: Use NRRD for the 6D ImageIO instead of MetaImage.
ENH: Use REQUIRED_FILES for the metric extractor test.

Avoid false positives on the dashboard when the dependent test does not
successfully complete.
Merge pull request #324 from vovythevov/MiscTubeTKFixes

Misc tube tk fixes
Merge pull request #323 from thewtex/metric-extractor-required-files

ENH: Use REQUIRED_FILES for the metric extractor test.
ENH: SegmentTubes test baseline after bug fixes in RidgeTraversal
Merge pull request #325 from aylward/SegmentTubes

ENH: SegmentTubes test baseline after bug fixes in RidgeTraversal
ENH: Improved estimation of SegmentTubes parameters
Merge pull request #326 from aylward/SegmentTubes

ENH: Improved estimation of SegmentTubes parameters