Handle ListedColormaps in expand and cmap_and_norm - #241

Description

Resolves #240 by checking whether colors is a ListedColormap directly upon entering cmap_and_norm rather than after modification (extension). Now works when defining the Style as follows, where levels represents the right border of each level and there is an additional extension at the end. I'm assuming that's the intended behaviour but not sure.

newstyle = Style(
    colors=ListedColormap(['#ff0100', '#feaa00', '#fffc03', '#ffffff', '#ffffff', '#eaccf8', '#af51c3', '#7a007b']),
    levels=[-2, -1.5, -1, 0, 1, 1.5, 2],
    extend="both",
)

# Plot with custom style:
fig = ekp.Figure(rows=1, columns=1)
subplot = fig.add_map(domain="Spain")
subplot.grid_cells(data, z="SPI1", style=newstyle)
subplot.legend()
fig.coastlines()
fig.show()

Testing without style= shows no change in the default behaviour (as intended) but I'm not sure what other tests are useful here.

Minimum working example, can probably be improved in terms of e.g. skipping parts of cmap_and_norm completely for ListedColormaps.

I think it would be good to add this to the documentation gallery somewhere but wasn't sure where. If you want to go really crazy you could add SPI and SPEI as built-in styles.

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