Use custom backgruond map

The GUI enables user defined background maps in .geojson format through the GUI constructor parameter String::map_boundary_file. One could for example download NUTS boundaries as GeoJSON from datahub.io, save this file at a desired location and use this file path as map_boundary_file.

Downloading NUTS2, one can with a EMX-case variable case

gui = GUI(case; map_boundary_file = joinpath(@__DIR__, "NUTS_RG_60M_2024_4326_LEVL_2.geojson"))

get something like