explore_model: DO preds & obs

on my wish list. can use streamMetabolizer::predict_DO(mymetab_model) to get predictions of both DO.obs and DO.mod; plot by grouping by date because dates are often overlapping or disjoint

Start by locating where model exploration and plotting currently enter the powstreams code, then review streamMetabolizer::predict_DO(mymetab_model) and its DO.obs and DO.mod outputs. Clarify the expected date-grouping behavior, including overlapping or disjoint dates; done should include both prediction series and a plot that presents them by date.

Written by the indexing model from the issue text.