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.
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
COMP: Fixing CppCheck warnings
COMP: Remove exec flag on MergeAdjacentImage ipython notebook
Merge pull request #327 from aylward/cppcheck

Cppcheck
ENH: Add routine to save displacement fields

from computed transformations.
Fix TRE file loading in vesselview

The previous commit was allowing normal TubeSpatialObject files to be
loaded but broke compatibility with VesselTubeSpatialObject files.

The problem is that an itk::TubeSpatialObject<3> isn't the same
as a itk::VesselTubeSpatialObject<3>::Superclass (i.e. the parent class)
since VesselTubeSpatialObject changes the point type used.
While the node was accepting both kind of tube types they do not share the
same base class (at least from a casting point of view), making it hard to
do the same thing for both classes with a clean code.

To remediate this in a simple way, we decide to only support
VesselTubeSpatialObjects in a vtkMRMLSpatialObjectNode.

Consequently modified itktubeTubeXIO to make sure it generates
VesselTubeSpatialObjects compatible files.
Merge pull request #329 from vovythevov/MiscTubeTKFixes

Fix TRE file loading in vesselview
Merge pull request #328 from ricortiz/SaveDisplacementFields

ENH: Add routine to save displacement fields
ENH: Update superbuild projects to make version used by Slicer
Merge pull request #330 from aylward/UpdateExternalProjectVersions

ENH: Update superbuild projects to make version used by Slicer
Fix ConvertTRE test baseline

Following 054b4b3de6be006b1526f71ffcdbf23ca37663ca, we need to update
ConvertTRE baseline to make sure the test compare with a correct tube file.

Change-Id: I6b01a2a8347cc517c688c123ba3f31b4ee94cd97
Merge pull request #331 from vovythevov/MiscTubeTKFixes

Fix ConvertTRE test baseline