feat: sibling stops transfers

[aubryio]

Fixes #59

I see value in distinguishing between generated "sibling stops" (stops that share the same parent) transfers and cross-stop transfers. In Switzerland, it's tricky to rely on the virtual transfers logic #73 as without more complex routing logic it would create transfers between the start and end stops of vertical elevators, steep funiculars, etc. as they have a very short (sometimes 0) travel distance.

Thus, I propose to:

• Give an option to create all missing sibling stops transfers and always use the default transfer time for those as they are often very close to each other, most likely with staircases and complex pathways making the actual distance hardly relevant.
• Use the optional "virtual" transfer logic on top, which would then focus on covering the remaining non-sibling stop to stop transfers.

@matthiasfeist don't hesitate to let me know if you see a use-case where this sibling stops transfers logic is an issue - we can alternatively make it optional too. EDIT: I made it optional via a profile config already.

I also added support for the "forbidden transfers" which gets applied to prevent explicitly forbidden transfers from being generated.

[matthiasfeist]

Nice one!