Handle GTFS feeds with pathway nodes

[abyrd]

This PR contains first steps to address issue #803. The most visible immediate effect is that it avoids spatial index errors from feeds with stops that lack coordinates when generating GTFS vector tiles. The underlying change is gracefully tolerating GTFS feeds that use the pathway extensions, even where we don't yet use those nodes in routing.

The code is already in working condition, but we may want to add more validation or build a full routable model from the pathways in transfers.txt. That could be done in this PR or a subsequent one.

[abyrd]

In the interest of getting this fix into the next maintenance release, we will not implement full pathway routing in this PR. In its current form the PR should now at least tolerate non-stop nodes in stops.txt and fully validate whether they must or must not have coordinates based on their types. In order to perform this validation, which requires checking internal references between rows in stops.txt and therefore cannot be applied on the fly as rows are loaded, this PR introduces post-load validation checks that run after all tables are loaded. Using this mechanism, this PR also includes a simple fix for #777: the calendar and calendar_dates tables are both treated as non-required, but a post-load rule adds a general error on the whole feed if no service entries were generated by loading the feed.

[ansoncfit]

I added a couple minor renaming suggestions. Otherwise, this looks good to me.