special case `percentage` in `combine`

[pdeffebach]

Currently combine special cases nrow to get the number of rows.

It's actually pretty hard to get the percentage of observations in a group when doing a combine operation. You have to do nrow before, then save that variable, then make a new nrow.

It might be worth considering adding something like

julia> df = DataFrame(
           sex = rand(["Man", "Woman", "Other"], 100), 
           race = rand(["White", "Black", "Other"], 100), 
           income = rand(100));

julia> combine(groupby(df, "race"), percentage)

be a shorthand for

N = nrow(parent(df))
n = nrow(sdf)
return n / N

[bkamins]

Actually it is even harder than you think, as gdf can be subsetted, so your code is incorrect, it should rather be:

n = [nrow(sdf) for sdf in gdf]
return n ./ sum(n)

which is not only not easy to type but also tricky.

I think it is OK to add this at some point.

[bkamins]

added :proprow