Makes XcFunc obey the general json interface used in pymatgen for easier serialization.

def as_dict(self):
        """
        Makes XcFunc obey the general json interface used in pymatgen for easier serialization.
        """
        d = {"@module": self.__class__.__module__,
             "@class": self.__class__.__name__}
        # print("in as_dict", type(self.x), type(self.c), type(self.xc))
        if self.x is not None: d["x"] = self.x.as_dict()
        if self.c is not None: d["c"] = self.c.as_dict()
        if self.xc is not None: d["xc"] = self.xc.as_dict()
        return d