[feature] Default forbid pickup/dropoff on last/first stop times of vehicle journeys

[woshilapin]

For trip planning to be handled correctly by Navitia, we need to forbid pick-up on the last stop time of each vehicle journey and drop-off on the first stop time of each vehicle journey. However, there is one specific case when we still want to leave the possibility of pick-up on the last and drop-off on the first: prolongation trips when the last stop of the first trip and the first stop of second trip are different.

• Squash/rebase the PR

ref: ND-991.

[woshilapin]

Some updates about this PR (removing the wip label as well as the Draft status).

• e18b4a2 is about improving documentation about the different cases, as well as making explicit the scenarios the current implementation does NOT handle.
• 32bd414 fixes all the fixtures so the tests pass
• f9bf9c4 is also about fixing the tests but it is a bit specific. It's part of Netex FR parser and is trying to generate Journey Patterns from multiple Vehicle Journeys. Bad luck, we had 2 different Vehicle Journeys in the test but the current PR was making these 2 Vehicle Journeys the same -> the tests were expecting 2 Journey Patterns but was getting one. I changed the second Vehicle Journey so it doesn't have the same Journey Pattern as the first.

[woshilapin]

A small improvement, I moved the .sort_by() later in the implementation, after removing all vehicle journeys that do not contain a block_id (call to .retain()). See 4a1563f.