Volume image inspector widget
Add an "inspection" operation to aeva whose parameters (an SMTK attribute) have a custom view using a new 3D widget that shows 3 axis-aligned slice planes. This should allow us to remove the slice planes that are currently part of the vtkSMTKResourceRepresentation class. The new widget should be implemented to allow users access to ParaView's colormap editor. The operation itself may either do nothing (inspection only) or perhaps allow painting on the slices to create voxel selections and edit segments.