Bug: `phenClones()` uses invalid sample size in permutation step

`phenClones()` errors during the permutation loop because it passes a vector to the `size` argument of `sample()`:

    ts <- sample(pvt$predicted.value, dim(mgd) * 2, replace = FALSE)

`dim(mgd)` returns a vector (c(n_pairs, 2)), so `dim(mgd) * 2` is also a vector, which leads to:
"Error in sample.int(...): invalid 'size' argument".

Probably need to change it `nrow(mgd)` instead