Full Williamson case 6 example

[JHopeCollins]

Now that we have the shallow water utilities, the new shallow water implicit solver, and paradiag.next_window(), we need an example for Williamson6. This example needs:

• solution and errors at 1, 7, and 14 days including calculating potential vorticity and CFL
• Information on the number of nonlinear and linear solves for varying mesh refinement and CFL
• weak scaling for constant window duration but varying number of steps per window
• ?? anything else?

[JHopeCollins]

This should use the update Williamson 6 initial conditions which introduce an explicit instability. This dominates over the numerical instability which can pollute the solution.