Number of default lanes is inconsistent with OSM documentation

[henrik-wolf]

The OSM Documentation on the lane tag (here) states, that this tag should be used to encode the total number of lanes (in both directions, if the way is oneway=false) available to motorized traffic. Looking at the defaults that have been chosen in LightOSM.jl, it seem to me like we think the lanes encodes only lines in one direction. I think we should rework (and better document) this and all other tags which get set as a default, as well as the algorithm which sets the default for the lanes tag. Maybe, we could also parse the lanes:forward, lanes:backward and lanes:both_ways, to clean up any ambiguity? I would be happy to take a shot at it, but the question of how to parse data, especially from OSM is always heavily opinionated, so I would like to get a little discussion started about how we would like to handle this problem, before I get stuck in.