algebraic multigrid

[gdmcbain]

I might make a start this week on algebraic multigrid.

As mentioned earlier this year, this will be useful for the momentum steps of the Uzawa Stokes solver #171. It's also required for all three steps of the pressure-correction scheme for the unsteady Navier-Stokes equation in gdmcbain/fenics-tuto-in-skfem#5. Of course, once in place it should be useful for much else too.

I guess the three candidates are PyAMG, pyamgcl, and PETSc. Probably one of the first two for most purposes as they're much lighter.

I might start with optional extensions to ex9, comparing AMG to ILU as a preconditioner for conjugate gradients on the Poisson problem.

I doubt this'll require any changes to the skfem library.

[gdmcbain]

Fixed by #273, extending ex09 to show how to use either PyAMG or AMGCL, both being good fits for scikit-fem. I'll defer PETSc.

[gdmcbain]

pyamgcl also worked well with the three steps of the ‘IPCS’ (incremental pressure correction scheme) for the unsteady Navier–Stokes equations in gdmcbain/fenics-tuto-in-skfem#5.