Formalize notion of an analysis object

[alexpiet]

I developed all of my code such that the dataframes I generated in the data_utils repo I add to the nwb object as attributes. I do this so its easy to keep track of multiple data frames from the same session (events, trials, fip_df, licks, etc). But its probably confusing to add it to the NWB object. So its probably better to make a class "analysis object" or something.