[boost] Median in Boost.Accumulators

Hello Boost Some time ago, I needed to compute the exact median for a stream of elements and after using Boost.Accumulators, realized that it only provides an approximation of the median and not the exact number. I rolled up my custom implementation of the algorithm and used it for my project. I wanted to check if people on this list might be interested in such an implementation for Boost.Accumulators in which case I can try to adapt my implementation and send a PR. Performance characteristics: For a stream of n numbers, the memory required is O(n). Median can be queried at any point of time in O(1) and adding all the elements costs O(nlog(n)) Lakshay