ENH: allow GeoDataFrame.dissolve(by=None)

[martinfleis]

Resolves #1380

If no column is passed to GeoDataFrame.dissolve(), whole GeoDataFrame is considered a single group and all geometries are dissolved to a single one. It is done using a __dummy column. Since aggfunc needs to be passed to groupby, we can't create gdf directly using unary_union and agg (e.g. DataFrame.agg('first) and DataFrame.groupby.agg('first) are different methods).

[jorisvandenbossche]

Looks good to me!

[knaaptime]

sorry for the resolution on this pic, but one other thing that is useful in this context is the ability to dissolve by polygon contiguity, i.e. resulting in 3 features rather than one multipolygon

i imagine you wouldn't want to add a dependency on libpysal to support this operation, but maybe the contiguity check could be done alternatively with pygeos?

[martinfleis]

maybe the contiguity check could be done alternatively with pygeos?

That would essentially reimplement fuzzy_contiguity from libpysal.

I think that this would be a great addition to examples gallery, maybe more than implementing it directly as a part of dissolve. If you don't care about aggfunc, you get contiguous polygons as dissolve().explode() and if you do, then there'll be that example :). Another reason is that Queen is still faster than fuzzy_contiguity (I think), so better to promote better solution.

[jorisvandenbossche]

This PR is ready to be merged, I think?