Rename distance_edge to distance_node_a on signal

[jeriox]

Signals currently have a distance_edge and a side_distance. Side_distance means the distance to the nearest point of the track (e.g. 1,5m left of the track). Side_distance is a somewhat misleading name, as it represents the distance from the signal to the Knoten_A of the respective TOP_Kante. Therefore, it should be renamed to distance_node_a. This requires changing all usages across the repositories of the ecosystem.

[jeriox]

erroneous distance_previous_node removed in #18