Extract all data points whose element centroid lies within the given
        polygon.

        Parameters
        ----------

        Returns
        -------

def extract_polygon_area(self, pid, polygon_points):
        """Extract all data points whose element centroid lies within the given
        polygon.

        Parameters
        ----------

        Returns
        -------
        """
        polygon = shapgeo.Polygon(polygon_points)
        xy = self.grid.get_element_centroids()
        in_poly = []
        for nr, point in enumerate(xy):
            if shapgeo.Point(point).within(polygon):
                in_poly.append(nr)

        values = self.parsets[pid][in_poly]
        return np.array(in_poly), values