duaIterate.py does not maintain flexible depart edge choice

[namdre]

when loading trips between TAZ (also junction-taz with fromJunction, toJunction or --mapmatch.junctions), the depart- and arrival-edge from the first iteration will be used for all subsequent iterations.

This is because the duarouter --alternative-output loaded in subsequent duaIterate iterations always writes routes and currently has no way to encode that a fresh rerouting attempt may start from a TAZ.

[namdre]

one a more technical level, option --write-trips suppresses --alternative-output