[Refactor]: create function for t=f(y,s)

[Rick-Methot-NOAA]

Refactor request

This equation is used repeatedly. Useful to turn it into a function.
t = styr + (y - styr) * nseas + s - 1;

e.g. t = get_t(yr,s)
FUNCTION int get_t(yr,s)

Expected behavior

more concise code