ModelFrame should support Abstract/SubDFs

[juliadata]

ModelFrame should support Abstract/SubDFs

[tshort]

I think I fixed this, but before this is closed, @HarlanH or @doobwa, please check. Tests passed, but I didn't test much beyond trying it on one sub(df).

[doobwa]

Several of these functions return DataFrames. If provided a GroupedDataFrame, should they return a GroupedDataFrame?

[tshort]

That's a good question, Chris. They probably should if they can. I didn't look in enough detail to see if it's possible. SubDataFrame is our only other AbstractDataFrame right now. For SubDataFrame's, there may not be that much difference between the approaches. We might know better once we have other AbstractDataFrame's like DDataFrame.

[doobwa]

Looks good for now, though we'll may revisit it once more AbstractDataFrame types are present.