Verify folder file format

        The folder file format is only valid when
        there is only one file format present.

def verify(path):
        """Verify folder file format

        The folder file format is only valid when
        there is only one file format present.
        """
        valid = True
        fifo = SeriesFolder._search_files(path)
        # dataset size
        if len(fifo) == 0:
            valid = False
        # number of different file formats
        fifmts = [ff[1] for ff in fifo]
        if len(set(fifmts)) != 1:
            valid = False
        return valid