include geojson serialization and geo_inferface

[mattijn]

This PR includes support for the __geo_interface__ on the topology object. Since the __geo_interface__ is build upon the geojson standard this also means there is now a .to_geojson() function.

The pretty and validate parameter will probably change later to False and False instead of the current True, True.

---

With the __geo_interface__ in place, visualization can be done directly using the Topology object in Altair:

Prepare a Topology object:

import geopandas as gpd
import topojson

df = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres'))
df = df[df.continent == 'Africa']

tp = topojson.Topology(df)
tp.toposimplify(1, inplace=True)

And then:

import altair as alt

# from this PR forward: with the __geo_interface__ serialization in place
# the serialization from TopoJSON to GeoJSON can also be done on the Python side
# btw: this happens within Altair
alt.Chart(tp).mark_geoshape().encode(color='name:N')

Compare this to the current approach (still and will be completely OK as well):

# conventional the serialization from TopoJSON to GeoJSON happens on the Vega side:
tp.to_alt(color='properties.name:N')

---

This PR also means that there will be two approaches to create a GeoDataFrame:

# conventional through fiona using the `TopoJSON` driver (under the hood)
tp.to_gdf().plot()

And now:

import json

# from this PR forward: using the .to_geojson() function
gpd.GeoDataFrame().from_features(
    json.loads(tp.to_geojson(validate=False))['features']
).plot()

Maybe the to_gdf() function should get a driver parameter, that can be set to TopoJSON or GeoJSON?

I will not close the relevant issues, since tests are not written yet.

[martinfleis]

Maybe the to_gdf() function should get a driver parameter, that can be set to TopoJSON or GeoJSON?

From my perspective, so a benchmark and implement in to_gdf() whatever is more performant. I would not complicate it with driver as the resulting GeoDataFrame should be exactly the same both ways. Keep in mind that you have to preserve CRS in the second example.