Reduce file size of a GTFS object
gtfs_compress(gtfs)a gtfs object
by default UK2GTFS tries to preserve id numbers during the conversion process to allow back comparisons to the original files, e.g. `transxchange2gtfs()` retains stop ids from the NAPTAN. However this means files sizes are increased. This function replaces ids with integers and thus reduces the size of the gtfs file.