I don't know how important it was that a `tTest` handle an arbitrary `TraversableOnce` of a `Fractional[T]`. Now it converts everything to doubles and does it's work.

My reasoning: if you have a `Seq[Rational]`, then whatever you do will be slow, and adding `numeric.toDouble` on top of that won't hurt much. On the other hand, you can get native array access performance if you use the `meanAndVariance` ufunc. 

I think maybe it'll be better to convert `tTest` to a ufunc anyway.