BUG: fix crash in volume rendering due to bad node type (multivolume)

The volume rendering node combobox allows child classes except for
ones known not to work. This fix adds the MultiVolumeNode to that
list in order to avoid a crash in the internals of the volume rendering
due to invalid number of components.

A better solution would be for the volume rendering internals to
gracefully refuse to render volumes that aren't suited. It would
be good if the volume rendering provided a method like ValidateInput
that would allow the application to let the user know that the
selected configuration won't work.

From: Steve Pieper <pieper@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24580 3bd1e089-480b-0410-8dfb-8563597acbee