Gauss Seidel

This can be parallelized as easily as Jacobi!

Phase I follows by Phase II.

Red black is a different ordering from original Gauss Seidel.

For (some) elliptic problems, can show red black as good as and maybe better than original ordering.

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i.e., red black does not need more iterations than pure Gauss Seidel.

For related hyperbolic problems-see NAS benchmark SSOR discussion in CPS713-not so clear.

For higher order differencing, need more phases (more colors) but principle the same.