Fix case surround repeating coords during quantization

[mattijn]

This PR fix #110.

After quantization the repeating coordinates in a linestring are filtered. But it should not happen that LineStrings become Points due to removal of repeating coordinates. This was correctly computed, but this case was not written back to the corresponding LineString.

Now this resolves correctly

import topojson as tp
import geopandas as gpd

gdf = gpd.read_file(gpd.datasets.get_path("naturalearth_lowres"))
gdf = gdf[gdf.name.isin(['France', 'Belgium', 'Netherlands'])]
tp.Topology(data=gdf).toposimplify(4).topoquantize(500).to_alt(color='properties.name:N')