Darcy solver implemented

The Darcy fluid solver is based on the paper "L. Goren, E. Aharonov, D. Sparks and R. Toussaint 2011 The mechanical coupling of fluid-filled granular material under shear, in Pure and Applied Geophysics", doi 10.1007/s00024-011-0320-4 .

The Darcy CFD solver is selected by having the parameter `cfd_solver = 1`. The standard value `cfd_solver = 0` selects the Navier-Stokes solver.

The iterative solution has also been improved for the standard Navier-Stokes solver.