Poisson example is broken

[tkphd]

• With np>1, the multigrid algorithm hangs. Implementation decisions suggest that parallel was not intended:
  • Ghostswap is never called, and
  • stock Gauss-Seidel iterative scheme is used where red-black Gauss-Seidel is more appropriate.
  • The multigrid algorithm may not be parallelizable.
• Serial code alternates between (-\infty, 0] and [0, +\infty) every timestep, with zeros along the border. This not the desired result.