BUG: More sides to tube SpatialObjectViewer rendering.

This applies and sets the number of sides to a higher value (32).  The default
could be decreased in the future.

This does not address of bug of setting the number of sides display property (which is
not getting applied).  The vtkTubeFilter default of 3 sides is only being
displayed without this patch.
ENH: Add ConvertTubesToSurface Application.

Convert a tubes .tre file (has points, radius, other descriptors at points)
into a file with a polydata representation of the tube surface.  Files can be
save in the .vtk legacy VTK file format or the .stl file format.  The .stl
file format can applied to 3D printers, for example.  The .vtk format also
retains some of the scalar data associated with the tubes.  It can be used to,
for example, visualize the tube Id's in Paraview and select sub-tube Id's.
STYLE: Const correctness and types in vtkMRMLSpatialObjectsStorageNode.cxx.
Merge pull request #202 from thewtex/tube-extension-sides

STYLE: Const correctness and types in vtkMRMLSpatialObjectsStorageNode.c...
Merge pull request #200 from thewtex/tube-render

BUG: More sides to tube SpatialObjectViewer rendering.
Merge pull request #201 from thewtex/tube-stl

ENH: Add ConvertTubesToSurface Application.