Basic statistical functions

This implements mean(), median(), var(), stddev(), and range().

I thought this would be simple... but I really ought to just stop thinking that, ever, in general, about anything. Here, I had not considered that implementing median() really requires a sort. So, now column-wise and row-wise radix sort is implemented, which will also be useful for sort() and sort_index(). I don't think my implementations are particularly blazing fast, but they work.