QuickSort Algorithm
1. If First < Last then begin
2. Partition the elements in the subarray First..Last so that the pivot value is in place (in position PivIndex)
3. Apply QuickSort to the first subarray First..PivIndex-1
4. Apply QuickSort to the second subarray PivIndex+1..Last
The two stopping cases are:
1. (First = Last) -- only one value in subarray so sorted!
2. (First > Last) -- no values in subarray so sorted!