Potential base temperature issues in the thermaltime and alternating model

[ypwong22]

Hi, I've been using the package and noticed a mismatch between the documentation and the code in the thermaltime and alternating models. Both models involved a term $max(T_i - threshold, 0)$, according to the documentation. In the code, this is implemented as gdd[gdd < threshold] = 0 , but this does not subtract the threshold from gdd. I changed this to gdd = gdd - threshold; gdd[gdd < 0] = 0. Please let me know if this modification is correct or not.