Include offset from node in calculation

[root676]

Possible to use spatialIndex methods to get the next network arc, then use QgsGeometry nearest point to determine the nearest point on the linestring, then cut linestring at this point and measure lengths to the beginning nearest node (starting point).

Simpler approach: just use crows distance and detour factor of 1.2 to the nearest node (QgsGeometry.shortestLine()).