int DOY of the growing season start according to the MTD.
Arguments
temperatures
vector Daily average temperatures in degree Celsius.
first_possible_DOY
int Only consider days of the year from this
value onward.
Details
This function implements the *multicriterial thermal definition* (MTD)
as described in chapter 2.3.1.3 of the dissertation of Andreas
Schaumberger:
Räumliche Modelle zur Vegetations- und Ertragsdynamik im
Wirtschaftsgrünland, 2011, ISBN-13: 978-3-902559-67-8