Theadwaysperformance: Function to return performance data based on headways...
In MBTAr: Access Data from the Massachusetts Bay Transit Authority (MBTA) Web API

Function to return performance data based on headways surrounding a given passenger's arrival time at a station on a given route.

1 2	Theadwaysperformance(enter_time, exit_time = NULL, enter_route_name, exit_route_name, enter_stop_name, exit_stop_name, data, api_key)

`enter_time`	Entrance time of passenger at station
`exit_time`	End of window in which to look for train arrivals.
`enter_route_name`	Route name for which headways should be returned.
`exit_route_name`	Route name of exit stop for which headways should be returned.
`enter_stop_name`	Stop name (alpha, non-numeric) designating which station at which train headways information should be returned.
`exit_stop_name`	Stop name (alpha, non-numeric) designating towards which station train headways information should be returned.
`data`	Dataframe with named variables given by other arguments.
`api_key`	API key for MBTA Performance API. To obtain one, visit the MBTA Developer Portal (https://mbta.com/developers/mbta-performance/)

`headway_mean`	Average headway during the time period given between 'enter_time' and 'exit_time'.
`headway_bench`	Average of benchmark headways during time period given between 'enter_time' and 'exit_time'.
`headway_perf`	Average delay (actual headway - benchmark headway) during the time period given between 'enter_time' and 'exit_time'.
`next_train`	Arrival time of next train at given station that is after the entrance time given with 'enter_time'.
`prev_train`	Arrival time of previous train to be at station on given route immediately prior to the entrance time given with 'enter_time'.
`headway_guess`	Difference (in seconds) between arrival time of next train and entrance time given with 'enter_time'.
`headway_bench_guess`	Benchmark headway time for the train arriving next after passenger's arrival. Divide by two to find benchmark wait time.