#142 alternate take, keeping semantics same

This is an alternate way to speed up #142. It's about 20% slower than the GH142 branch PR, because nest and groupby are two separate operations ...

    open Deedle
    open System
    
    let genRandomNumbers count =
      let rnd = System.Random()
      List.init count (fun _ -> (float <| rnd.Next()) / (float Int32.MaxValue))
    
    let r = genRandomNumbers 1000000
    let x = [for i in r -> if i > 0.5 then "x" else "y" ]
    let s = Series.ofValues(x)
    let n = Series.ofValues(r)
    let f = Frame.ofColumns ["s" => s]
    f?n <- n
    
    let means = f.GroupRowsBy<string>("s") |> Frame.nest |> Series.mapValues (fun df -> df?n |> Series.mean)