Parse the first occurence of func_name in the XML file.

def _parse_radfunc(self, func_name):
        """Parse the first occurence of func_name in the XML file."""
        node = self.root.find(func_name)
        grid = node.attrib["grid"]
        values = np.array([float(s) for s in node.text.split()])

        return self.rad_grids[grid], values, node.attrib