Benchmarking the first functional checkpoint

• With constant gravity, and explicit time stepping (dt=1.0 seconds) an isothermal/constant-gamma atmosphere is stable.

• With imex time stepping (dt=300, tstep_type=7), a constant-gamma atmosphere is stable (though w is of magnitude 1e-5 after 1200 seconds. w explodes near model top.)

• Enabling/disabling remapping does not affect stability for explicit time stepping.