More class creation

[dajmcdon]

I already wrote an epi_recipe wrapper around recipe. This performs 2 functions specific to our use case. First, the template is always a single row to avoid copies. Second, it handles time_value and geo_value (and other keys) properly.

1. Do we implement a epi_workflow around workflow. Current workflow does not support post processing of any type. And it doesn't have many useful S3 methods (residuals, fitted, etc).
2. Do we implement wrappers around predict to keep the keys.
3. epi_recipe doesn't currently add a class (it has class recipe). Do we add one?

We need to do something to solve 2.