Make multidim upwind more forgiving of bad grids.

Now it will no longer trigger assertation failure when grids are not
edge-conformal (faulted cornerpoint grids processed by our code will
usually not be). Minor algorithm change to handle this.
Optimize multi-cell solve and add new Gauss-Seidel variant.
Removed experimental multi-cell solver code.

Since the Gauss-Seidel approach seems to be both simplest and
fastest, all parts dealing with assembling multicell systems
have been removed.