'as_dataframe' option belongs in reader functions, not in TableBundle.__init__()

`TableBundle.__init__()` currently has an `as_dataframe` parameter. 
It is rather incongruous. 
This parameter would seem to better belong in the reader functions, as one more option for their `to` parameter i.e. `to='dataframe'`. That would seem to harmonize well with the other uses of `to`, and rid us of the awkward asymmetry in the `TableBundle.__init__()` signature and implementation. 