Allow returning geometry by leg

[TheMarex]

Issue

Right now it is kind of a pain to get the geometry by leg. You can request the steps and accumulate it, but that is annoying if you are not interested in the instructions. With hindsight that wasn't the greatest API decision, alas better fix it late then never. In general it doesn't make sense to have it by route and by leg at the same time, so it seems appropriate to extend the overview option for triggering the new behavior.

This is still work-in-progress, I need to spend some time to make sure the flatbuffer stuff isn't broken by this and add tests.

Implementation

• [x] Add a new overview=by_legs parameter that will add the (unsimplified) geometry by leg.
• [ ] Add support to flatbuffer API

Tasklist

• [ ] CHANGELOG.md entry
• [ ] update relevant Wiki pages
• [ ] add tests (see testing documentation)
• [ ] review
• [ ] adjust for comments