This fixes a long-standing problem in the computation of the RHS of the potential equation in Grid_SolveForPotential.C. This was pointed out by Dan Reynolds last year, but I didn't work out the ramification and the fix until now. The basic problem is that Grid_SolveForPotential.C assumes the cell spacing on the grid is L/Nx, while the multigrid solver assumes it is L/(Nx-1). As Dan points out, this means that we end up solving a problem of the form
Laplace(Potential) = (G/a)(GravitatingMass)(dim1-1)(dim2-1)(dim3-1)/dim1/dim2/dim3
as opposed to:
Laplace(Potential) = (G/a)*(GravitatingMass)
I spent some time figuring out to change the multigrid solver, but that turned out to be a bad idea, since it works best with that cell spacing. Instead, it is much easier to adjust the RHS computation. Note these changes are completely separate from the recent discussion about depositing subgrid cells.
This will require a regeneration of the gold standard.