Metro lines

[fnattino]

City like Paris and Berlin have outdoor metro lines, which we should include in the network as well

[cforgaci]

@fnattino should this issue move to the CRiSp repo? Or should we keep it as a prototyping task if we are not sure it is needed?

[fnattino]

Maybe good to keep it here as it is not specific on the CRiSp package but more on experimentations and applications? But it is true that depending on the results we might need to update CRiSp defaults..