ENH: Temporary fix for contour slice intersections

Based on today's discussions, a type check has been added to model displayable manager to facilitate proper contour display until the new segmentation node object is created and a new displayable manager is added.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23372 3bd1e089-480b-0410-8dfb-8563597acbee