Add barotropic jet test group

[scalandr]

This test group implements the barotropic jet test from An initial-value problem for testing numerical models of the global shallow-water equations (https://www.tandfonline.com/doi/abs/10.3402/tellusa.v56i5.14436). The initial condition consists of a basic zonal flow, representing a typical mid-latitude tropospheric jet, with a correspondingly balanced height field, plus a small unbalanced perturbation to the height field; the perturbation is used to induce the development of barotropic instability. The test group is composed of three test cases:

• A steady-state case: the model is solved for 6 days with the zonal jet and the unperturbed, balanced
  height field as initial conditions. This step is useful to ensure that the unperturbed zonal flow is properly balanced.
• A short adjustment case: the perturbation is added to the balanced height field and a short simulation is run (6h) to examine the gravity wave dynamics.
• A barotropic instability case: a longer simulation is run with the addition of the perturbation to the balanced height field to investigate the vortical dynamics.

[xylar]

@scalandr and @alicebarthel, just wanting to check in on this work. Are either of you finding time to push forward on this?

[cbegeman]

@scalandr and @alicebarthel When you have time, can you provide an update on whether the 3 test cases are producing the results that you wanted and perhaps add the plots that this test case has produced as well? Just trying to get a sense of whether this work only needs a rebase or more substantive development work.