Add finite depth parameter to SQG model (surfaceqg.jl)

[mncrowe]

I suggest adding a new depth parameter, say H, which can either be $\infty$, giving the current model, or finite.

In the finite case, the only change is that the Fourier space operator linking b and ψ changes from

$$ \hat{b} = |\textbf{k}|\hat{\psi}, $$

to

$$ \hat{b} = |\textbf{k}| \tanh(|\textbf{k}| H)\hat{\psi}. $$

From looking at the code, I think this just means changing the nonlinear function calcN_advection!, though some of the diagnostics might also need minor changes depending on how they're calculated.

Happy to help implement this.

[glwagner]

Nice! It might make sense to build this feature as an option to precompute this function and store it in an array. That would be more flexible and also may save computational cost since tanh are relatively expensive.