Support geospatial data conversion

[daslu]

Both R and the JVM have a good story for processing geospatial data.

R has sf, Java has JTS, etc.

Communicating between the two has a good potential for Clojure. For example, it will allow for easy, declarative geospatial visualization with R libraries such as Leaflet, the related Mapview and the new Mapdeck.

This issue suggests to make geospatial data conversion between R and the JVM officially supported by Clojuress.

For most use cases, simple Geojson represetntation is good enough as a way for communication. This could be used in a first version of the implementation.

For a first step of this task, it would be good to write an example workflow with explicit conversion using Geojson, even if it is not yet part of the r->clj and clj->r API functions.

Some experiments have shown this is a practical way, at least for non-huge datasets.

edit: wording