DOI-USGS
improve plot_DO_preds for pi, oipi, and sim models
for y_var %in% c('conc', 'pctsat'), pi models should have prediction line segments that each start at a DO.obs value and progress toward the next one. what we have now is what might be called the DO.pure predictions - what we get if we extrapolate the daily metab params to DO.mod assuming no error. oipi models should have the state-space estimates in DO.mod, which are pi-corrected but don't need to be line segments starting at DO.obs values.
with bayesian models we can track inst estimates of err_obs_iid and err_proc_iid, which could help with both pi and oipi plotting.
may want to plot both DO_mod (the state, which is DO.obs for pi models) and DO_pure (aka DO_det for 'deterministic'?). And what about DO_mod_partial, which is deterministic based on the previous timestep's state?
This function could also use updating for GPP-process-error models (pp, err_proc_GPP=TRUE).