extract stats from OnlineStats

[simonbyrne]

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One of the problems I hit with the TrueFX notebook is that there wasn't a clear way to extract the numerical values from OnlineStats objects. I ended up doing map(value, tbl) but it seems like there should be a clearer way to do it (and figure a way to give the columns names as well).

[joshday]

In the other issue from @shashi:

I think the default should be value...

That makes sense for Mean and Variance, but makes less sense for things like Hist (where you probably want to use the object to make a plot). I'd say we should return the value but need an option to opt out and get the OnlineStat itself. I think adding a keyword arg for reduce and groupreduce would be sufficient. Thoughts?

[shashi]

Yeah, that sounds perfect. A keyword argument will do the job.