Add permutation tests from predictmeans

[pitakakariki]

These are robust to non-normal errors. Maybe need to add permutation as an alternative to simulation?

[pitakakariki]

Run comparisons using:

1. small samples
2. non-normal data

[pitakakariki]

Fitting robust models might be more effective than applying robust inference to standard models (see https://github.com/pitakakariki/simr/issues/271). Permutation might be a good default for "robust simulation".