Trait for significance testing

[larsgw]

I was thinking of adding a trait like FrequentistTest:

trait FrequentistTest {
  fn p_value(&self) -> f64;
}

It would automatically support something like is_significant(a: f64). However, mosts tests so far (and probably most, if not all tests in the future) will produce an estimate that lies somewhere on a distribution. At that point it might make more sense to have the trait require (f64, T) where T: ContinuousCDF and have the trait deal with alternative hypotheses (#1).