Added median to std.algorithm

[JackStouffer]

Split from #3592

I will propose the same question I had in the other PR

I also want to add various other statistical methods to Phobos, such as standard deviation, which fit in std.algorithm, and things such as rolling mean, which don't fit into std.algorithm. I was thinking that something like a rolling mean would best fit in std.numeric, but, would it be best if all of the statistical methods were in the same module, or is it ok to have them separate?

[9il]

Ditto - #3679 (comment)

[DmitryOlshansky]

s/mean/median/

[DmitryOlshansky]

Otherwise looks good to me. One thing though - a range may not be sorted but you may have called nth_element algorithm placing r.length/2 element in its place while leaving the rest unsorted.

This way one may have median of a range w/o sorting it.

[JackStouffer]

https://github.com/9il/phobos/pull/3