The function calculates the daily heat unit accumulation (GDH)
from hourly temperature data, using a standard linear model or the
linear model proposed by Anderson and Seeley (1992). The standard
model is defined by a base temperature, and the Anderson and Seeley
(1992) includes also optimum and critical temperatures. In both
variants, heat accumulation begins when temperatures are above a
minimum (base temperature, Tb), and growth increases linearly with
temperature. In the Anderson and Seeley (1992) variant, growth no
longer increases once the optimum temperature (Topt) is reached,
meaning that GDH above it are constant. The critical temperature
(Tcrit) is the temperature above which growth ceases (i.e. GDH=0).
The function allows the user to define Tb, Topt and Tcrit, and uses
as default the values set by Anderson et al (1986) for fruit trees:
Tb=4ºC, Topt=25ºC and Tcrit=36ºC. In the standard linear model with
upper thresholds, use Topt = 999 and Tcrit = 999.