Debug Vissum import issues

[gmatosferreira]

Following #80, @temospena tryes to import the generated GTFS to the Vissum software and there are some issues that were raised during this process:

• Stop area missing because no stop

• stop_times not imported because no stop point

[temospena]

Also, ferry PuT is not matching any OSM information

[temospena]

Manually updating in the Transport Systems set, and redifining in all links (and opposite direction) with Ferry.

[gmatosferreira]

The problem with the missing stop areas seems to be related to the fact that tidytransit::filter_feed_by_date method that we use to filter the GTFS before converting it to OSM shapes and performing the metropolitan unification is leading to the loss of the parent stations (stops$parent_station) from the stops table of the GTFS feed due to the fact that the tidytransit method only considers stops that actually have services at stop_times, which is not the case.

I opened a PR at tidytransit suggesting an implementation to fix this: r-transit/tidytransit#231

Let's hope they accept it. 🤞