Change weather data representation

`Weather` is now a type that stores the complete simulation data
(previously it just stored the weather data for a single day).
Weather data storage is now a struct-of-arrays, previously it was a
dict-of-structs.

Weather data can be accessed with functions such as `sunshine(weather,
date)`.

Missing weather input data for temperature (min/max/mean),
precipitation, and evapotranspiration is now an error.  In the future,
missing values could perhaps be imputed.

`AgricultureModel` and `Weather` are now defined with `@kwdef`,
allowing their constructors to be called with keyword arguments.