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Mutates the duration of activities randomly within a specified range.
For the first act, the end-time is mutated. For the last act, duration and end-time are set to UNDEFINED.
For all other acts, the duration is mutated, and the end-time is updated accordingly.
If an activity has no duration set (UNDEFINED_TIME), a random time between 0 and 24h will be chosen.
Departure and arrival times of legs are updated according to the activity durations, but the
leg travel time is not modified (e.g. updated according to new expected departure time).