Relaxation factor of the relaxation operator used for the
multigrid method. This is mainly for debugging,
since overrelaxation does not help in a multigrid scheme.
The default is 1.0, except 0.6666 for the gauss_jacobi method.
Method used to solve the linear system approximately in each grid for the
multigrid procedure that solves Poisson equation. Default is gauss_seidel,
unless curvilinear coordinates are used, in which case the default is gauss_jacobi.
Alternative implementation of Gauss-Jacobi.