Vertical advection of u,v + tendencies (vor, pres grads)

[milankl]

This implements the vertical advection for $\xi = u,v$ both simultaneously.

$W_k(\xi) = -\frac{1}{2\Delta p_k} \left( M_{k+1/2} ( \xi_{k+1} - \xi_k) + M_{k-1/2}(\xi_k - \xi_{k-1} \right)$

So mass flux $M = p_s\dot{\sigma}$ (on half levels above and below $k$) times the full level difference across respective half level mass flux.