routine to generate balanced initial fields for SWE

[jshipton]

Routine to initialise shallow water flows from a given vorticity field such that the initial flow is non divergent and has div(u_t)=0. The steps are:

1. Compute the streamfunction from the vorticity.
2. Compute the velocity from the streamfunction.
3. Solve a mixed Poisson equation for (v, depth) where v=u_t and the right hand side comes from the vector invariant form of the velocity equation and setting div(v)=0.
4. Adjust the depth to have the correct mean.