Add support for Envelope geometry objects

[mvanbommel]

Currently, envelope geometry objects are not supported in the app. Addressing this issue will involve identifying when a data set uses envelopes, and appropriately plotting those envelopes on the map.

Contrived example data: https://developer.here.com/documentation/studio/map_customization_suite_hlp/dev_guide/topics/geojson-basics.html

If you know of a real dataset that uses envelopes, please add it in the comments.