Control the length of the output of the variance function

At least in simple cases, the output of the variance function should be of length the number of columns of the Y variable in input.

- [ ] Determine whether this has to be true in all cases
- [ ] If so, try to test it at the variance definition step (it may need a dry-run of the variance function) or at the execution step (easier)

MWE (thanks to @ThomasDeroyon) :

```r
data <- data.frame(id = 1:100, sex= c(rep(1, 50), rep(2, 50)),
y = c(rnorm(50, 1, 3), rnorm(50, 2, 4)), weight = rep(10, 100))

# Dont't work
variance_function <- function(y) 1
variance_wrapper <- gustave::define_variance_wrapper(variance_function = variance_function,
reference_id = data$id,
reference_weight = data$weight,
default_id = "id")
variance_wrapper(data, as.factor(sex))

# Works
variance_function <- function(y) rep(1, NCOL(y))
variance_wrapper <- gustave::define_variance_wrapper(variance_function = variance_function,
reference_id = data$id,
reference_weight = data$weight,
default_id = "id")
variance_wrapper(data, as.factor(sex))
```