Implement an Arrange method for DataFrames

[kniren]

One should be able to sort the DataFrame by one or several of it's columns.

func (d DataFrame) Arrange(keys string...) DataFrame { ... }

A possible implementation could start by enabling each Series to return a []int array containing it's sorted order. For example:

a := Strings("b", "c", "a")
var b []int = a.Order() // b == []int{2,3,1}

In case we have NA elements we should decide what to do with them. Maybe they all have the same order index and appear at the end?

a := Strings("b", nil, "c", nil,"a")
var b []int = a.order() // b == []int{2,4,3,4,1}

In any case, once we have an []int array for each key column we could calculate the new row order array and use it to sort the DataFrame.

[kniren]

DataFrame.Arrange and Series.Order are implemented as of 65850ab

No matter the direction of ordering, NaN elements will always be sorted at the end.