Regression / integration tests for `ocean_simulation`

[glwagner]

As discussed on #126, we currently only have the long-running example to test the performance and correct behavior of the code. However, the example takes 4 hours to run and is inappropriate for CI.

We therefore need additional tests --- either regression tests, or integration tests that create and export plots (if needed) for ocean_simulation. Ideally these will run in less than 1 hr.

cc @simone-silvestri