initial_cohort_fork_length: Initial cohort fork length
In fishsciences/YoloBypassSBM: Yolo Bypass Salmon Benefits Model

Initial cohort fork length based on model day and Chinook run

1	initial_cohort_fork_length(chinook_run, water_year_string, date_index)

`chinook_run`	Run timing classification: Fall, LateFall, Winter, Spring
`water_year_string`	Water year (1997-2011) as a string
`date_index`	Index of date in a water year that cohort enters the model; in all years except WY1997, equivalent to day of water year