Implement weather-dependent transmission model

[atmyers]

This implements a model where temperature and humidity data affect disease transmission. The formula used is:

$$ p(T,AH) = p_{max} \exp(-\beta_{AH} * AH - \alpha_T * \max(0,T - T_0)) $$

where the meaning of the parameters is described in the documentation. To turn this on, add agent.do_weather=1 to your inputs file.