Create a custom widget to include controls

[keewis]

This widget is an alternative to #197, but does not support the player nor does it use H3HexagonLayer for h3 cells. The main advantage of having a single widget is that it will make #196 much easier.

This PR also splits up the plotting module into multiple files, which may make working on the code a bit easier.

[relativityhd]

Looks promising! A few features I would miss as a user from #197 :

• "Robust" colorscaling via 2nd and 98th percentile (This is actually very important to me, also to having it enabled by default)
• Per-Variable colorize settings
• Type hints for the explore method of the accessor

[keewis]

"Robust" colorscaling via 2nd and 98th percentile

This should be done in the most recent commit. Let me know if you'd need this also for centered plots (i.e. where center is set, we call abs(var).max() to infer the limits right now).

[relativityhd]

Yes, I think it would make sense to have it also in the centered version, at least I implemented it in #197

[keewis]

I've added everything except per-variable colorize options, which I will punt to a future PR (same as the special casing of H3 for plotting).

I believe the failing docs are caused by the widget breaking the map composition examples (merging and side-by-side), but this can be taken care of in #196

[keewis]

(also, if you notice anything that's wrong / broken, I don't mind iterating quickly)