StopTime Aggregator

StopTime structs are not meaningful individually but only when they're grouped by trip. The `stop_times.txt` file is also not guaranteed to be sorted. It is very useful to have a helper that does this automatically.

I propose a struct Run and a supporting iterator whose constructor groups and sorts the stoptimes for you:
```rust
#[derive(Debug, PartialEq)]
pub struct Run
{
pub trip: &Trip, // or just trip_id String
// StopTime is verbose, maybe create a new struct with less fields?
pub sequence: Vec
}
```