Add information about a node describing a critical point.

        :param idx: unique index
        :param unique_idx: index of unique CriticalPoint,
        used to look up more information of point (field etc.)
        :param frac_coord: fractional co-ordinates of point
        :return:

def _add_node(self, idx, unique_idx, frac_coords):
        """
        Add information about a node describing a critical point.

        :param idx: unique index
        :param unique_idx: index of unique CriticalPoint,
        used to look up more information of point (field etc.)
        :param frac_coord: fractional co-ordinates of point
        :return:
        """
        self.nodes[idx] = {'unique_idx': unique_idx, 'frac_coords': frac_coords}