AtmosphereMap visualization: Replace scatter plot

[valentinaschueller]

Idea: a loop over latitudes and a call to pcolormesh for every latitude.
But first: Wait how OpenIFS NetCDF output is loaded by Iris.

[valentinaschueller]

We have the same issue with the NetCDF output as before with GRIB output: Iris is not able to load the grid in a structured way and we have kept on creating a scatter plot with the atmosphere output data. @JanStreffing suggested that we could try using Matplotlib's tricontourf(). This might be a good compromise between the scatter plot and writing (hacking) a custom plotting routine for this type of grid.