finish migrating some functions to gtfstools

[rafapereirabr]

We have already migrated quite a few function to the gtfstools package. I'm creating this issue so we can organize what functions we have already migrated (and so we could remove them from gtfs2gps), and what functions we could still migrate.

• filter_by_agency_id()
• filter_by_day()
• filter_by_route_id()
• filter_by_route_type()
• filter_by_shape_id()
• filter_day_period()
• filter_single_trip()
• filter_week_days()
• merge_gtfs_feeds()
• remove_by_route_id()

[rafapereirabr]

The filter_single_trip() will be kept for internal use in the gtfs2gps

[rafapereirabr]

We're waiting the new version pf gtfstools to hit CRAN so we can close this issue