Time traveled between stops tests

This adds tests for the Time Traveled Between Stops indicator.

It also changes the indicator itself slightly. After a discussion with @jbranigan, we agreed to change the indicator so that it calculates only travel time, rather than travel time + dwell time. This is closer to how the indicator is described in the PDF, although the original SQL didn't do it this way. The change doesn't make a major difference in the numbers for SEPTA (~4 seconds was the maximum change I could find), although it could for a system that had long dwell times.