Add side-by-side map comparisons of runs and baselines to the showcase

[dnerini]

Add side-by-side map comparisons of runs and baselines to the showcase

New showcase.comparisons config section and plot_compare_maps rule that
plot each ML run next to selected baselines at the same valid time, with a
shared colour scale. Defaults: disabled, alps domain, SP_10M, and the lead
times produced by both the run and every selected baseline.

Move get_style() and preprocess_field() from plot_forecast_frame.py to
src/plotting/styles.py so both scripts share them.

Example output:

[Louis-Frey]

Hi Daniele,
Nice functionality, thanks! I'm testing the PR currently (feedback maybe next week).
At some point, Carlos was working on something similar, have you picked up from him?
Have a nice weekend!