Account for the case of no shapes.txt file in transit length indicator

This also includes a modification that was needed due to a GTFS parser interface change.

Note 1: there is still one more case that hasn't yet been accounted for: when there is no shapes.txt file, but there are shape_distance_traveled values in stop_times.txt. The only GTFS feed I was able to find that fits this description fails to parse using the GTFS parser. Will need to wait until that's addressed to cover this final case.

Note 2: The GTFS parser isn't transparently using UTM for lengths, so the calculations are currently being done in WGS84. Support for UTM will need to be explicitly added, which is outside the scope of this task.