[boost] Adding 'indirect_sort' to boost::algorithm

At Boostcon last month, there was a discussion of indirect sorting algorithms to boost::algorithm. An “Indirect sort” (called 'argsort' in numpy - https://numpy.org/doc/stable/reference/generated/numpy.argsort.html) returns a sequence of indices into the collection describing how the collection can be rearranged in order to become sorted. So I created https://github.com/boostorg/algorithm/pull/117 to start down this road. (There’s clearly more that could be done here - stable_sort, partial_sort, nth_element, etc) Comments welcomed. — Marshall