code: def compress(self, delete_tif=False, folder=None):
          
          return compress(self.images, delete_tif, folder)
docs: Lossless compress all images in experiment to PNG. If folder is
          omitted, images will not be moved.
  
          Images which already exists in PNG are omitted.
  
          Parameters
          ----------
          folder : string
              Where to store PNGs. Defaults to the folder they are in.
          delete_tif : bool
              If set to truthy value, ome.tifs will be deleted after compression.
  
          Returns
          -------
          list
              Filenames of PNG images. Files which already exists before
              compression are also returned.