Fix combination of flex access and constrained transfer

[hannesj]

Summary

Currently RAPTOR crashes when combining a flexible access with constrained transfers. This PS fixes that

Unit tests

Added unit test for this feature combination.

Code style

Code style followed

Documentation

Nonen required

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[t2gran]

The FLEX access, especially the arrival on-board, create a lot of exception cases in the Raptor routing logic, optimized transfer and path mapping. This bugfix add tho this. I am not so happy about this and think I want to look into a better way of encapsulating this logic. I do NOT think we should do it as part of this PR, but have it in mind when we look at improving the heuristic. We might even drop some of the Raptor variants at some point - making the changes a little easier.

The previous() method is not used in the inner routing loop and the previousTransit is only used when finding constrained transfers, so I am not to worried about the degrade in performance for these changes - even if they do change som of the core logic.

[leonardehrenfried]

Looks good. I fixed a tiny typo though.