Bugfix: state phi should be initialised to zero in multigrid solve

When solving with the multigrid solver, the initial solution should be set to zero (this is because in the loop solver we currently solve the residual equation and in CG we really do a solve starting from zero).
Improved way of setting omega / detailed maxiter specification

Calculate omega by working out the grid-spacing on the finest level explicitly from the number of cells. Specify maxiter separately for the outer- and inner- iteration.