Add geojson support to map widget

[rambip]

This pull request adds experimental geojson support for the map widget.

This fixes #158

The current design choice is the following:

• keep Longitude and Latitude as required columns, with the asumptions that most users will only want the old behaviour
• add an optional GeoJSON column. If set, this will take precedence over Latitude and Longitude. The format is the "geometry" key of the geojson standard, and supports any type.

I tried to keep the original behaviour when possible, and adapt it to geojson:

• The geojson tiles are also clickable, when you click inside the region. When you click it, it changes the opacity.
• the map bounds are computed using the points in the geometry
• when there is an error (like: geoJSON and Lat / Long in the same column), an error is displayed. I was not able to see the error even with the original code.

Something I would like to add is a way to specify colors, because it can be very useful for polygons and lines. But it may be outside the scope of this PR.

Note: the formatting is pretty odd at some locations, it would be nice to format the code consistently. I use the typescript formatter with default settings.

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[rambip]

@berhalak any progress on the subject ?

[berhalak]

Hi @rambip,

Sorry for the delay. Will review this week.