Stratified permutation test

[kellieotto]

Design:

• pass in string OR function, just like in two-sample function. Pass this to the helper function
• need to specify a function to combine across strata

Somehow, these need to depend on the alternative. For two-sample, the alternatives are greater, less than, or not equal. We may also want to consider ordered alternatives or differ.

Pass a triple (test statistic, combination function, alternative) into the function?

We also want to think about naming conventions.