Adding median reduction

It would be great to have a reduction operator that computes a median. Here's a first pass:

https://gist.github.com/kidpixo/16e76ac201375e4d3fb4bf89b8a9b6ad

(Notes adapted from @jcrist:) Unfortunately, median is one of the few reductions that can't be parallelized trivially. In the code above, self.column is just a string name, not the column itself, hence the error shown. Implementing median will require custom append, combine, and finalize methods, so it will be fairly complicated. There are various parallel median algorithms findable on google, but it's not clear that they would apply in this case.