support for multiple gdf and geojson as input

[mattijn]

WIP. This PR aims to support multiple geodataframes as input to store it in a single topojson file.

Eg. combination of global continent-level and country-level data.

[mattijn]

BTW. Reading of topojson files that includes multiple objects is already supported:

with open(r'tests/files_topojson/gm.topo.json') as f:
    topo_file = json.load(f)
    
topo_nl = Topology(topo_file, object_name='nl_features')
topo_gm = Topology(topo_file, object_name='gm_features')

topo_nl.to_alt(projection='mercator')

topo_gm.to_alt(projection='mercator')

[mattijn]

Goal is to support the following approach:

import geopandas as gpd
from topojson import Topology

world = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres'))
world = world[['continent', 'geometry', 'pop_est']]
continents = world.dissolve(by='continent', aggfunc='sum')

Topology(
    data=[world, continents], 
    object_name=['world', 'continents']
)

[mattijn]

I like this feature. Check the following:

import geopandas as gpd
from topojson import Topology
import altair as alt

africa_countries = gpd.read_file(gpd.datasets.get_path("naturalearth_lowres")).query('continent == "Africa"')
africa_continent = africa_countries.dissolve(by='continent', aggfunc='sum').reset_index()

topo = Topology(
    data=[africa_countries, africa_continent], 
    object_name=['countries', 'continent']
)

# disable dataset consolidation
# see https://altair-viz.github.io/user_guide/data_transformers.html#dataset-consolidation
alt.data_transformers.consolidate_datasets = False

# simplify the topology in once for both objects and visualise 
topo.toposimplify(5, inplace=True)
topo.to_alt(object_name='continent') | topo.to_alt(object_name='countries')

[mattijn]

Doc here: https://mattijn.github.io/topojson/example/input-types.html#list-of-geodataframes

[mattijn]

Accidentally commit to master instead from a PR. This commit e32cfc3 provides support for multiple geojson objects as input.
Documented here: https://mattijn.github.io/topojson/example/input-types.html#list-of-geojson-objects