arrival_time and departure_time should be optional

[medwards]

According to the spec if a stop_time is Timepoint::Approximate then arrival/departure_time doesn't have to be present (it's up to the consumer to interpolate the times).

If the timepoint column isn't present at all then technically I think that at least the first and last stop_time of trip must have times but this seems like too much validation at this point on basic transit structs (maybe when Runs are introduced).