Decide whether to define val time period in _targets.R

[lekoenig]

Currently, the time period used for model validation is defined in two places, _targets.R and the model yml file. These two locations also currently contain different time periods, so we should simplify how we define the val times.

One option is to just omit the val times from _targets.R. Those val times do get used in 2a_model.R and are used to create visuals. One downside I see with omitting these variables from the targets file is that in this pipeline _targets.R serves as a config file, at least for the data processing steps. I think we can introduce confusion by defining variables separately in the model config files, although a comprehensive README for running/modifying the modeling steps would help with that (see this issue).