Plot HEALPix grid with earthkit-plots

[SF-N]

This PR shows how to plot a HEALPix grid with earthkit-plots, which is possible since version 0.3.1, cf. ecmwf/earthkit-plots#37.

[juntyr]

Thanks! Could you extend the color map levels to make the earthkit plot less coarse? Could we also keep the second pyicon plot or is there a reason to axe it?

[juntyr]

(I will likely integrate this PR into #36 tomorrow)

[SF-N]

Thanks! Could you extend the color map levels to make the earthkit plot less coarse? Could we also keep the second pyicon plot or is there a reason to axe it?

I tried to extend the levels by setting the levels argument in quickplot but this didn't change the result, so this doesn't seem to be used here and I also didn't find an other way to make it look smoother. I would keep it as it is with the default settings unless you have an idea.
I added the pyicon plot again, but please note that this notebook is quite large, when executing it more than 2 times, I get a memory error.

[juntyr]

Blocked on ecmwf/earthkit-plots#67