Feature request: dissolve a la GeoPandas

[asinghvi17]

This requires us to have good unary unions in GeometryOps. But it allows aggregating high-granularity datasets, going from counties to states for example.

Maybe this just gets solved by DataFrames groupby/combine though. Then we need tutorials for it.

[asinghvi17]

This currently "works" with Julia API:

zones_cleaned = DataFrames.combine(groupby(DataFrame(zones), :LocationID), :geometry => x -> LibGEOS.unaryUnion(GI.GeometryCollection(x)))

but it needs to be significantly cleaner!