BUG: Fixed loading of RGB volumes

SingleFile flag was often incorrect in saved scenes, because some file formats store a volumetric image in multiple files,
while in another format the same data can be saved in a single file; but the singleFile flag was not updated when file format
was changed. This caused loading error for RGB volumes when there were multiple files with similar names in the same directory.

Solution: Don't save SingleFile flag to the scene (it is just a temporary hint for file reading anyway), but set its value
based on GetNumberOfFileNames() value.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25692 3bd1e089-480b-0410-8dfb-8563597acbee