Make sure RISK is never displayed as negative

The amount of viable released spores is strongly inhibited by solar radiation. The spore load is also reduced by precipitation and by germination . Survival factor of released spores : set SFRS(t)=(1-(Qo(t)-270)/540)/1,5 - inhibition by radiation

SFRS will be negative when Q0 (solar radiation) is > 810 w/sqm. This means that VRS will be negative, which in turn makes RISK negative. We change the setting of Result objects so that all negative RISK values are converted to 0.0.

This has been done in cooperation with the current responsible person for the model, Haavard Eikemo https://nibio.no/ansatte/havard-eikemo