code: def find_DQ_extension(self):
          
          dqfile = None
          dq_suffix=None
          if(self.maskExt is not None):
              for hdu in self._image:
                  # Look for DQ extension in input file
                  if 'extname' in hdu.header and hdu.header['extname'].lower() == self.maskExt.lower():
                      dqfile = self._filename
                      dq_suffix=self.maskExt
                      break
  
          return dqfile,dq_suffix
docs: Return the suffix for the data quality extension and the name of
          the file which that DQ extension should be read from.