Tiny little edit to deal with float calculations

Basically, you'll sometimes have covariates that are completely
identical for all members of the subgroup so they shouldn't be included.
However, when you demean those values you might end up with very small
numbers for everyone rather than exactly 0, so the column's standard
deviation isn't exactly 0. This change just looks for very standard
deviations rather than SDs that are exactly 0.