set_downstream_recursive: Recursively set downstream stations
In ECCC-MSC/Basin-Delineation: Finds upstream area and upstream stations

recursively sets all stations downstream of a target station

1	set_downstream_recursive(station, table, next_downstream, set_to = 1)

`station`	target station to set
`table`	a data.table describing upstream / downstream relationships between stations.
`next_downstream`	the next station downstream of the target station
`set_to`	toggle between setting upstream (1) or not upstream(0)

M  <- initialize_matrix_from_stations(c('stn1', 'stn2', 'stn3', 'stn4', 'stn5'))
set_upstream('stn1', M, c('stn2', 'stn3', 'stn4'))
set_upstream('stn2', M, 'stn3')
set_downstream_recursive('stn5', M, 'stn3')
get_downstream('stn5', M)