run log should store model objects as a column for further interaction(s) about bbr

[seth127]

Having the model object as a column in run_log() output could still be useful in theory, but the use case described here is solved by #29

Worth noting though, the summary_log() output does contain the full bbi_nonmem_summary object in it's own column, which is analogous to the proposed change here. Just pointing out that we do support this pattern elsewhere so maybe it makes sense to do it here too, for the sake of consistency.

[seth127]

Would be able to simplify model_summaries.bbi_run_log_df dispatch if we did this.
https://github.com/metrumresearchgroup/rbabylon/blob/develop/R/model-summaries.R#L113 if we did this.