def _compute_cell_circumcenters(self):
        """Computes the center of the circumsphere of each cell.
        """
        # Just like for triangular cells, tetrahedron circumcenters are most easily
        # computed with the quadrilateral coordinates available.
        # Luckily, we have the circumcenter-face distances (cfd):
        #
        #   CC = (
        #       + cfd[0] * face_area[0] / sum(cfd*face_area) * X[0]
        #       + cfd[1] * face_area[1] / sum(cfd*face_area) * X[1]
        #       + cfd[2] * face_area[2] / sum(cfd*face_area) * X[2]
        #       + cfd[3] * face_area[3] / sum(cfd*face_area) * X[3]
        #       )
        #
        # (Compare with
        # <https://en.wikipedia.org/wiki/Trilinear_coordinates#Between_Cartesian_and_trilinear_coordinates>.)
        # Because of
        #
        #    cfd = zeta / (24.0 * face_areas) / self.cell_volumes[None]
        #
        # we have
        #
        #   CC = sum_k (zeta[k] / sum(zeta) * X[k]).
        #
        alpha = self._zeta / numpy.sum(self._zeta, axis=0)

        self._circumcenters = numpy.sum(
            alpha[None].T * self.node_coords[self.cells["nodes"]], axis=1
        )
        return
Computes the center of the circumsphere of each cell.