Do smart things by default

[ljwolf]

Recently, I've been finding that I like the following "style" of legendgram interaction with continuously-varying classifications coming from geopandas:

legendgram(f,ax, data, np.percentiles(data, q=(1,*np.arange(10,100,10), 99), ...)

where the palettable palette is chosen to be something with 11 categories matching the map.

I think we should make this happen if no breaks are provided.

[ljwolf]

addressed in #11