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 #324 from vovythevov/MiscTubeTKFixes

Misc tube tk fixes