Add SetSpatialObject method to directly update a node from a filename

This prevent the hassle of having to add a node to the scene, copy it and
then delete it. It's also cleaner as we don't have to deal with that new
node display nodes. Before we were leaving them around after the node's
deletion which is bad for perfomances.
Also add a Reset method to the spatial object node. This allows a bit of
code refactoring and improves the nodes API.
Merge pull request #307 from vovythevov/SimplifyTRELoading

Add SetSpatialObject method to directly update a node from a filename
ENH: Testing (and fixing) IO methods related to TubeExtractor and TubeX
COMP: Conversion of i/o fstream arg to char * from string
Merge pull request #308 from aylward/TubeIO

ENH: Testing (and fixing) IO methods related to TubeExtractor and TubeX
ENH: ConvertTRE and ConvertTubesToImage w/radii
Merge pull request #309 from aylward/TubeIO

ENH: ConvertTRE and ConvertTubesToImage w/radii