Missing tests for norm=Normalize(...)

[Sonja-Stockhaus]

Currently, we don't have enough tests including the norm=Normalize(...) argument. It should work for matplotlib and also datshader (after #309), but I think we need some more tests and maybe also a bit more documentation.

Also, I noted that both seem to give the same result, not sure if that's a bug

from spatialdata.datasets import blobs
from matplotlib.colors import Normalize
blob=blobs()

blob.pl.render_points(color="instance_id", size=40, norm=Normalize(4, 7, clip=True)).pl.show()
blob.pl.render_points(color="instance_id", size=40, norm=Normalize(4, 7, clip=False)).pl.show()