This tutorial demonstrates how to solve the linear system in a
canonical ABecLaplacian form

    alpha * a * phi - beta * del dot (b grad phi) = rhs.

Here phi is the unknown in a cell-centered MultiFab, alpha and beta
are scalar constants, a is a cell-centered MultiFab, and b lives on
cell faces and is thus represented by D face based MultiFabs, where D
is the number of spatial dimensions.  The right-hand side, rhs, is
also cell-centered.  A more specialized version of the ABecLaplacian
form is Poisson's equation.  This tutorial is written with AMReX's
Fortran interfaces.

After the solution is obtained, one can also ask for grad phi, or flux
(defined as -b grad phi).  Note that they live on cell faces.  This
step is not performed in this tutorial.  However, one can look at
`amrex/Src/F_Interfaces/LinearSolvers/AMReX_multigrid_mod.F90` for the
interface of `get_grad_solution` and `get_fluxes`.

