Interface, handling doubles and NaNs

[derfugu]

I think to be useful the algorithm would have to be able to handle NaNs and floating points. I played around with the tester.cc a little bit. What I needed was a function

Vector slidingMedian(Vector x, int h) which returns the sliding medians over x for a given window length h. I tweaked the code a bit to work with doubles and expose that function (I hard wired the sorting algorithm to sort, that may be an (optional) third parameter). But then I realized it cannot handle NaNs and that's a dealbreaker in practice.