Calculate daily vernalisation and accumulate to g_cumvd

Calculate daily vernalisation and accumulate to g_cumvd

wheat_de_vernaliz_days(maxt, mint, tempcr, g_cumvd, dlt_cumvd)

Arguments

maxt

Daily maximum Temperature

mint

Daily minimum temperature

tempcr

Crown temperature

g_cumvd

cumulative vernalisation days till yesterday

dlt_cumvd

delta cummulative vernalisation days