Enhancement: add per-route check for rounded stop_times

[landonreed]

After discussion with @abyrd and @evansiroky of #145, we have concluded that the PR fixes the primary issue (abounding validation issues for zero hops), but that there could be feeds (e.g., a merged feed for an entire nation) that contain routes which treat rounded stop_times differently and we could still amass a large number of zero hop validation issues.

A future improvement might be to keep track of rounded stop_times on a per-route basis and only store a validation issue for rounded stop_times for individual routes. So the different scenarios for feeds could be:

1. all stop_times rounded (feed-wide validation issue)
2. some n routes with all stop_times rounded (n route-level issues, perhaps a handful of zero hops were flagged)
3. no routes with all stop_times rounded (perhaps a handful of zero hops were flagged).