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Some subtour consist of only a single trip, e.g. if a trip starts and ends on the same link or facility.
By default, for those modes, the normal chain based modes are available. But in certain cases, not all
the modes should be available for such trips (e.g. car-sharing does not make much sense for such a trip),
thus the list of modes available for single-trip subtours can be specified independently. As mentioned,
it is initialized by the constructor to the full list of chain based modes.