Test Case 4 - Forced Low in a Mid-Latitude Jet Stream
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Experiment 4 tests the full equations with semi-implicit time stepping.
Contour plots of the height, eastward wind, northward wind, divergence and
vorticity field and vector plots of the wind have been enabled 
on an orthographic projection.
Momentum forcing was enabled so that the low moves eastward in the flow.

WARNING: this case has very likely a bug in it at the moment  - The errors
are significantly too large !

