[MAINT] integrate `splot.mapping.mapclassify_bin` into mapclassify

[slumnitz]

Outlined in this conversation the mapclassify_bin functionality should be moved from splot into mapclassify:

• [ ] Integrate mapclassify_bin into mapclassify.
• [ ] Integrate tests into mapclassify
• [ ] rename to something more meaningful
• [ ] change dependencies in splot where former mapclassify_bin was needed.

ToDo: Not all mapclassify classifications are currently supported, look into changing API to support all (?)