Add add_vector_layer function

[giswqs]

Fix #437

This PR adds:

• add_vector_layer function for loading any geopandas supported vector format, including shapefile, geopackage, kml
• vector_to_geojson function for converting any geopandas supported vector format to GeoJSON
• Update the notebook example to include the add_vector_layer examples.

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📚 Documentation preview: https://jupytergis--445.org.readthedocs.build/en/445/
💡 JupyterLite preview: https://jupytergis--445.org.readthedocs.build/en/445/lite

[github-actions]

👈 Launch a Binder on branch giswqs/jupytergis/vector

[github-actions]

Integration tests report: appsharing.space

[martinRenou]

I guess this will download the GeoJSON if I do something like:

url = "https://github.com/opengeos/datasets/releases/download/world/countries.geojson"
doc.add_vector_layer(path=url, name="GeoJSON")

This is unfortunate since it will embed the GeoJSON data into the JGIS file, which may be an unwanted behavior. Keeping the GeoJSON source pointed by URL may be better.

So I guess we'd want to consider two approaches in this function:

• if it's a URL, we keep the URL as-is and don't embed the data
• if it's a local file or some in-memory data in Python, we don't have the choice but to embed the data so we keep this logic here