Vorticity Example from MHS

[jpfairbanks]

This is a discretization of the incompressible Navier Stokes equations using the Discrete Exterior Calculus.

The formulations are based on those given by Mohamed, Hirani, Samtaney (in turn from Marsden, Ratiu, Abraham).

However, different choices in discretization are chosen for purposes of brevity, to demonstrate novel discretizations of certain operators, and to demonstrate the automated Decapodes workflow.

[jpfairbanks]

I had to comment out Cahn_Hilliard.md to get the docs to build locally. We can remove that page in this PR, and then fix it in another PR.

[GeorgeR227]

I think the problem was that a new release of Decapodes hadn't been put out in a while. I think Cahn Hilliard should work on the latest.

[lukem12345]

Need to squash for next time