Explore existing GeoJSON extension

[deeplook]

Apparently there is a Jupyterlab GeoJSON extension, but its output cannot be used in ipywidgets: https://github.com/jupyterlab/jupyter-renderers/tree/master/packages/geojson-extension. OTOH, the GeoJSON renderer in ipyrest returns an ipywidget and can also be used in classic notebooks. But maybe the Jupyterlab extension contains something reusable...