This function sorts the array 'index' destructively. That means
that the array itself is changed and returned, no copy is created.
If extra arguments are given, they are supposed to be arrays of the
same size. Each of these arrays will be modified in the same way as
'index'. Ie. if index 3 is moved to position 0 in 'index' index 3
will be moved to position 0 in all the other arrays as well.
Sort can sort strings, integers and floats in ascending order.
Arrays will be sorted first on the first element of each array.
Sort returns it's first argument.