clean the zone system to avoid zones that are not connected to the network

[todo]

https://github.com/msmobility/mito_long_distance/blob/b88149f01689887f903e2a6b43a4a1d5a74a7471/src/main/java/de/tum/bgu/msm/longDistance/airportAnalysis/AirTripsGeneration.java#L629-L634

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This issue was generated by todo based on a TODO comment in b88149f when #9 was merged. cc @msmobility.