Consider adding viewsource and sourceindices functions

[bkamins]

This is a follow up from discussion in #2325 to keep track of it.

parent and parentindices applied on SubDataFrame or DataFrameRow give indices in the data frame that actually stores data that the view points to (currently it is only DataFrame). This is the same behaviour as is in Julia Base.

However, for data frames it might be useful to also produce viewsource and sourceindices that gives the same information but for a direct source from which SubDataFrame or DataFrameRow was created (similarly to what rownumber does for DataFrameRow).

Adding this would complicate a design a bit and at lest I believe that it is not really needed (there were no requests for such functionality; as opposed to rownumber which I believe is useful). Therefore I leave this issue open to discuss and get feedback if this functionality is needed.