Saves a constructed Morse-Smale Complex in json file
            @ In, filename, a filename for storing the hierarchical
            merging of features and the base level partitions of the
            data

def save(self, filename=None):
        """ Saves a constructed Morse-Smale Complex in json file
            @ In, filename, a filename for storing the hierarchical
            merging of features and the base level partitions of the
            data
        """
        if filename is None:
            filename = "morse_smale_complex.json"
        with open(filename, "w") as fp:
            fp.write(self.to_json())