BarotropicModel and ShallowWaterModel with several layers

[milankl]

At the moment the number of vertical levels nlev can be chosen for BarotropicModel and ShallowWaterModel but the layers aren't actually coupled (BarotropicModel: independently executed, ShallowWaterModel: Only the first layer is part of the time integration). I believe for the BarotropicModel we can do Charney and Phillips, 1953

for every layer. This is interesting because it suddenly couples the layers through stream function, although vorticity remains the only prognostic variable in each layer. I believe that could be easily set up, even for the n-layer system. At the moment we don’t calculate the stream function explicitly, as it’s not used anywhere else, but thanks to #142 the operators are easily available. We could cut the algorithm to get from vorticity to stream function and from stream function to velocities into two parts to get the numerator. That looks like what’s described in section 3 as method B.

@maximilian-gelbrecht