The covariates. Defaults to NULL. In this case, all
numeric traits in .data, except that in resp are selected. To
select specific covariates from .data, use a list of unquoted
comma-separated variable names (e.g. traits = c(var1, var2, var3)),
an specific range of variables, (e.g. traits = c(var1:var3)), or
even a select helper like starts_with("N").