Enhancement: dealing with sex differences in groups

[AMChalkie]

This may well be overkill, however it is still the case that we sometimes end up with sex differences in our samples. The option of being able to label the samples as M/F, and having that go into the fit would be really nice.

[drpowell]

I think the right thing to do is create a "hidden factor" for one of the sexes. This will then go into linear model.