Add vorticity and statistics to output

We need to add vorticity as an optional output. This is going to be another 3D field so will have to consider any memory storage issues.

We also need to add statistics: mean velocities $\\bar{u}\, \bar{v}, \bar{w}$, cross-velocity moments $\\bar{uv}, \bar{uw}, \bar{vw}\$ and velocity fluctuations)