nge: investigate ways to refactor NGE to ease the constraints (times, round trips)

[louisgreiner]

Main issue: https://github.com/osrd-project/osrd-confidential/issues/923

Exploration plan : B -> C -> D

Preliminary for asymmetry

The asymmetry is supposed to be set on a Trainrun, to allow on the same network to have symmetrical and asymetrical trainruns.

B - Enable trainrun section departure/arrival assymetry at nodes

The idea is to get rid of the following rule: a trainrun section arriving on a node at x, departs again from this node at 60 - x at the return trip. In other words, targetArrival + targetDeparture = 0 % 60.

C - Enable one-way trip

D - Enable different travel times for round trips

E - Enable different itinerary for the way back of a round trip (not on the scope of this issue)

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With @Yohh and @Uriel-Sautron

[louisgreiner]

First draft of B at SchweizerischeBundesbahnen/netzgrafik-editor-frontend#407 (PR closed, branch still available here)

/!\ At this stage, the symmetry is broken and we get warnings : it is normal since with the same travel time for both ways and non-symmetrical times at source/target node, only 1 travel time is not enough to determine a trainrun section (see TrainrunsectionValidator in NGE code) -> I disabled the warnings not in a proper way, might need to work on it