add `forecast_date` as a first-class part of the recipe object

[dsweber2]

Currently the forecast date is scattered across several functions. After #370, we should include it as a part of recipe construction in the first place, so we don't have multiple places where it might be added that might contradict eachother.

[dajmcdon]

@dsweber2 Do you have an implementation of this somewhere? I think it would be helpful for the step_climate_predictor() implementation.