Remove DataFrame assumptions from Expr

[mrocklin]

Eventually we'll have all sorts of Exprs like arrays, bags, etc.. Today we're mostly focused on dataframe and that's good I think.

However, even today we have non-DataFrame exprs like scalars, and the results of to_parquet calls. Maybe it's time to pull out some of the dataframe metadata assumptions like divisions and maybe even meta from the Expr class and make some new Frame class (or some better name) from which dataframe-like-things (dataframe, series, index) inherit.

[phofl]

I think this makes sense doing now if we already run into other expressions. I can take a look at this.

[mrocklin]

Just to make my current thinking concrete, I'm guessing that we have something like the following:

class Expr:
    def simplify(...)
    def operands(...)
    def __dask_graph__(...)
    ... def other things that have to do with graph manipulation

class Frame(Expr):
    def divisions(...)
    ...  other things that have to do with dataframe metadata
    def groupby(...)
    ... other dataframe API methods

Probably it gets more complicated once someone gets more deeply into this topic though.

[phofl]

Yeah that was my understanding as well.

I'll take a look and see whether I can come up with something useful.