Add Pressure Gradient design document

[sbrus89]

This PR adds the design document for the pressure gradient module for Omega-1. Compiled version here.

Checklist

• Documentation:
  • Design document has been generated and added to the docs
  • Documentation has been built locally and changes look as expected

[vanroekel]

Looks good, just a couple tiny questions from me.

[philipwjones]

Just a couple of questions...

[mark-petersen]

Test compiled and posted here.

[xylar]

@sbrus89, a few things to chat about at our Wednesday meeting (or sooner if you prefer). Nothing that should hold up moving forward on this.

[xylar]

What is $b$ in this case?

[xylar]

I think maybe this should be fleshed out a bit more. What analytic test would we use?

[sbrus89]

I revised this part slightly. I think we just need to specify arbitrary functions for the $\alpha$, $h$, and $z$, then we can test the convergence of the discrete tendency compared to the analytical solution.

[mark-petersen]

Latest version compiled here.

[mark-petersen]

Thanks for your work on this. It looks great!

[xylar]

A few comments on the design doc, all of them small and hopefully easy to address.

[xylar]

@sbrus89, I don't remember where things stand but this needs to be made consistent with the revisions to the v1 equations in #351.

[xylar]

I'm approving but I think we might still want to remove the baroclinic gyre test as not planned.

[hyungyukang]

Found a typo while reviewing #353

Suggested change

	\alpha \nabla_z p &= \alpha\left( \nabla_r p + \rho g \nabla_r z\right) + \nabla_r \Phi - \frac{\partial r}{\partial z}\frac{\partial gz}{\partial r} \nabla_r z, \\
	\alpha \nabla_z p + \nabla_z \Phi &= \alpha\left( \nabla_r p + \rho g \nabla_r z\right) + \nabla_r \Phi - \frac{\partial r}{\partial z}\frac{\partial gz}{\partial r} \nabla_r z, \\