Growing season indices: start, end, length

Index functions for calculating start/ end of growing seasom according to ETCCDI definition, with the exception that spatial dependency is not implemented ( i.e. the indices are only valid for the northern hemisphere).

gsstart: The first occurrence of at least 6 consecutive days with tas > 5 °C

gsend: The first occurrence after 1st July (Jan 1st in SH) of at least 6 consecutive days with tas < 5 °C.

gsl: Growing season length, calculated as the difference between gsend and gsstart