Implement Calov and Greve (2005) integrand

This allow positive degree days to be calculated assuming a normal distribution of temperatures around a mean value.
Added interpolation and integration

They are used by the computation of snow precipitation (accumulation) and positive degree days. Interpolation reduces the model sensitivity to whether a single month's temperature falls just below or just above freezing.
Improve artificial climate

This is a bit more interesting as there are places where temperatures never reach above, or below the freezing point.
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