Use GeoJSON for log export

[johannessen]

Spatial data serialisation using JSON should preferably follow the GeoJSON standard (RFC 7946), for which widespread support exists in mapping software such as QGIS or Leaflet.

Implementing GeoJSON log export by hand shouldn’t be too difficult. But apparently there are also multiple Python libraries available:
https://duckduckgo.com/?q=geojson+python

[cr]

Let's not add external dependencies if it can be avoided. GeoJSON seems like a simple enough format, so just organize the data structure as expected by the format and output it with the builtin json module.

My general stance on adding data formats is that there needs to be a very compelling reason to go beyond supporting one widely supported standard. hxtool is supposed to focus on talking to the radio, and we already have GPX support, so external tools should be able to take care of conversion to and from other formats.