[boost] [sorting] Multikey quicksort implementation

Earlier, I posted asking whether users would like a class that would be appended onto the end of their objects to signal that the array is over. However, that is not possible, as when appended to the end of the array, it will be treated as if it were of the same type as everything else in the array, therefore, it would be undetectable. The only other way to do this is to have users append a certain value to the end of each array that is guaranteed to be unique. This should not pose a problem for many applications, as there is almost always going to be one unique value, and in many cases, there are values that are not allowed as input into an array, making it easy to find a unique value.