gtfstools (development version)

New features

New function convert_stops_to_sf().
New function convert_shapes_to_sf().
New function filter_route_type().
New function filter_route_id().
New function filter_trip_id().
New function get_parent_station()
New parameters to read_gtfs(): fields, skip and encoding. The warnings parameter was flagged as deprecated.
New parameters to write_gtfs(): files, standard_only and as_dir. They substitute the previously existent optional and extra, which were flagged as deprecated. The warnings parameter was flagged as deprecated too.
Not exported: copy_gtfs_diff_field_class(), convert_from_standard(), convert_to_standard()

The get_trip_speed() and set_trip_speed() examples, tests and vignette bits now only run if {lwgeom} is installed. {lwgeom} is an {sf} "soft" dependency required by these functions, and is listed in Suggests. However, package checks would fail not so gracefully if it wasn't installed, which is now fixed.
Fixed a bug in which the crs passed to get_trip_geometry() would be assigned to the result without actually reprojecting it.
Changed the behaviour of get_trip_geometry() to not raise an error when the 'file' parameter is left untouched and the GTFS object doesn't contain either the shapes or the stop_times table. Closes #29.

Some utility functions previously provided by {gtfs2gps} will now be exported by {gtfstools}. Huge thanks to the whole {gtfs2gps} crew (Rafael Pereira @rafapereirabr, Pedro Andrade @pedro-andrade-inpe and João Bazzo @Joaobazzo)!
The package now imports {gtfsio}, and many functions now heavily rely on it, such as read_gtfs() and write_gtfs().

Functions no longer validate GTFS objects on usage. validate_gtfs() will be flagged as deprecated as well, since I plan to heavily change its usability and outputs in future versions.
write_gtfs() parameters went through major changes - the optional and extra params were flagged as deprecated and substituted by the more general files and standard_only.