Document compositional fields and other aspects better.

Dunno, I might be wrong. But I'm thinking like this:

0 = div(rho u) 
= rho div(u) + u.grad rho 
= rho div(u) + u drho/dp.grad p
= rho div(u) + u drho/dp rho g

Consequently,
div(u) = -u drho/dp . g

But the minus sign is missing on the right hand side of the equation. Think of it this way: if a pocket of material flows down, then u and g are collinear and u drho/dp.g>0. We expect the pocket of material to compress, so we expect the divergence to be negative. 

Or am I dense?