The class Multiscale_sort<Sort> represents a sorting algorithm adaptor. Given a range of n points:
  1. it applies Sort on the last (1 -ratio) × n points,
  2. it recurses on the first ratio × n points, stopping when there are less than threshold points.


Multiscale_sort<Sort> o ( const Sort &sort = Sort(), int threshold = 1, double ratio = 0.5);
constructs an instance with traits as traits class instance.


template <class RandomAccessIterator>
void o.operator() ( RandomAccessIterator begin, RandomAccessIterator end) const
sorts the range [begin, end).
Requirement: Sort::operator()(RandomAccessIterator begin, RandomAccessIterator end) is defined.