Add bbox on the index, add topoquantize and chaining of toposimplify and topoquantize

[mattijn]

This PR adds the ability to apply topoquantization (topoquantize). Therefor it was necessary to have bbox as key in the object.

Also chaining of the toposimplify() and topoquantize() functions is possible now.

This is coded as follow:

import geopandas
import topojson

data = geopandas.read_file(geopandas.datasets.get_path("naturalearth_lowres"))
data = data[(data.continent == "Africa")]

tj = topojson.Topology(data, options={'prequantize':1e6, 'topology':True})
tj.toposimplify(2.9).topoquantize(1e4).to_alt()

Or interactively using ipywidgets:

from ipywidgets import interact, fixed
import ipywidgets as widgets

style = {'description_width': 'initial'}
eps = widgets.FloatSlider(min=0, max=10, step=0.1, value=0, description='Toposimplify Factor', style=style)
qnt = widgets.FloatLogSlider(value=1e6, base=10, min=1,max=6,step=0.5, description='Topoquantize Factor', style=style)

def toposimpquant(epsilon, quant, topo):
    return topo.toposimplify(epsilon).topoquantize(quant).to_alt()

interact(toposimpquant, epsilon=eps, quant=qnt, topo=fixed(tj))

Gif output: