Saves object to a file in YAML format.

        Args:
            filename (str): Filename.

def to_file(self, filename):
        """
        Saves object to a file in YAML format.

        Args:
            filename (str): Filename.

        """
        d = {"mass_info": self.mass_info,
             "nonbond_coeffs": self.nonbond_coeffs,
             "topo_coeffs": self.topo_coeffs}
        yaml = YAML(typ="safe")
        with open(filename, "w") as f:
            yaml.dump(d, f)