Add roaming bikes

[farao]

This adds (next to stations) the roaming_bikes list to BikeShareSystem and
implements it for Nextbike (seems to be the only currently supported
provider that has roaming bikes that you can park/get outside of
stations).

[farao]

Any thoughts on this? :-)

[Altonss]

Will this PR be merged? I'm just curious, because it is now a year old :)

[eUgEntOptIc44]

@Altonss @farao

please also see #495 it's related to this issue. Regarding nextbike's 'flexzones' please see here: https://sharedmobility.github.io/Nextbike.html#flexzones (geojson files)

For those who read this and wonder what a 'flexzone' is: it's the area within roaming_bikes roam.

Hope @eskerda or possible other maintainer(s) might have the time to look into this any time soon.

[theRoboxx]

Is there an update on this request and what is the current problem?
Normally, I use this website for flexzone information on devices that don't use the google play services and therefore can't display the map in the nextbike app.

[eskerda]

Adding roaming bikes is not a specific nextbike thing, but affects other networks, so we need to consider all implementations when adding this feature to pybikes. Changes on this will also affect the infra where we run pybikes to generate the API, so it's not that trivial. We use lat/lng to generate ids, and roaming bikes change lat/lng constantly, so adding this without planning will backfire. Some networks generate a different id too every time a bike is used for anonymity, so there's that too.