Function to read weather data and format them for maize.model
maize.weather(working.year = NA, working.site = NA,
weather_all = weather_FranceWest): year for the subset of weather data (default=NA : all the year)
: site for the subset of weather data (default=NA : all the site)
: weather data base (default=weather_FranceWest)
data.frame with daily weather data for one or several site(s) and for one or several year(s)