improve predict_DO output for pi, oipi, and sim models

see also #294 (for plots specifically): DO predictions for non-oi-only models require different methods from oi-only models.
- [ ] `predict_DO.metab_bayes` should make use of `err_proc_iid`, `DO_mod`, etc. if these are tracked and stored; otherwise they should give a message or warning describing how close we can get, or not, with the available information
- [ ] `predict_DO.metab_model` and `mm_predict_DO_1ply` use err.obs=err.proc=0 right now. they should use errors determined during the model fit, shouldn't they?