Keyword arg `cols` and `source` for `mapreduce`

[haberdashPI]

Just learned that reduce has support for cols and source when called with vcat. However, mapreduce does not. Would be nice to have it!

[bkamins]

Unfortunately it is impossible to have it in general see #3016.
The reason is that your source data would have to be data frames and you would have to ensure that the map part of the mapreduce also returns data frames, which cannot be checked in general.

So I am closing this request (but if you have some idea how to resolve the issues mentioned in #3016 please comment and I will re-open)