Get the window center (level) and width values from an SoDataRange node

Get the window center (level) and width values from a DICOM volume. If the query fails, returns false and sets 'winCW' to 0,0. Uses the Window Center (0028,1050) and Window Width (0028,1051) tags from the first image in the stack. If these tags do not exist in the volume, then window center and width are computed from the actual data min and max values. (Note that querying the actual min and max values may take some time because every voxel must be loaded.) This method may be called with a non-DICOM volume. In that case the actual min and max data values are used.