Online diagnostics for vertical velocity

[roxyboy]

For a layered system, we can get the vertical velocity from the governing equation of layer thickness, which is then related to the stream function. I think it may be nice if we could output this as an on-line diagnostic.

[bderembl]

now done offline in https://github.com/bderembl/qgutils

for future reference, it is also possible to diagnose w from the buoyancy tendency as shown below with p0 and p1 two outputs that are one time step apart (with a very small dtout)

b0 = qg.p2b(p0,dh,1/Ro)
b1 = qg.p2b(p1,dh,1/Ro)

dbdt = (b1-b0)/dtout

p0_b = qg.interp_on_b(p0,dh)
ugradb = qg.jacobian(p0_b,b0, Delta)

DbDt = dbdt + ugradb

w_diag = -DbDt*Fr[:-1,..]**2