Making OSM structs blueprint readable

[p-schulz]

I think this could be of interest for some people working with this great plugin.

I'm currently working with UE4.18, so I can't modify the plugin for 4.19... But maybe someone else could make the structs in StreetMap.h (or more their UPROPERTIES) BlueprintReadOnly? I locally added BlueprintCallable getter functions in StreetMapComponent.h for retrieving the properties of buildings, roads and their nodes. This way it's easy to iterate over all roads and create splines for them (see #38). Due to OSM roads consisting only of single segments, the roads would look much nicer with splines.

(Test map with spline meshes)