docs: def add_overlay_to_slice_file(
          self,
          filename,
          overlay,
          i_overlay,
          filename_out=None
      ):
          """ Function adds overlay to existing file.
          """
          if filename_out is None:
              filename_out = filename
          filename = op.expanduser(filename)
          data = dicom.read_file(filename)
          data = self.encode_overlay_slice(data, overlay, i_overlay)
          data.save_as(filename_out)