Add travel time performance indicator

This commit includes indicator calculation parameter additions which make possible the use of observed data as well as the 'on time performance' indicator/tests.
I'm not sure that it's necessary to have the trips divided up by SamplePeriod; trip ids are supposed to be unique, so I think that a Map[String, Trip] would be sufficient here. That might simplify things; it would allow `params.observedForTrip(period, trip.id)` below to become something like `params.observedTrips[trip.id]`. I might be missing something though.