Dwell Time Indicator

This adds a Dwell Time performance indicator; there is a bug / problem with the Observed data which is that if an observed stop time is shifted too much, it can fall outside of the SamplePeriod. Currently, if this happens, the Indicator will throw an exception and fail horribly; I tried switching to using Options but that didn't play well with the current GTFS Parser. The hack to fix this is to pad the SamplePeriod by 2 hours in either direction to pick up any shifted stop times, but that's not guaranteed to work. Suggestions for better fixes welcome.