Obtain in matrix form the OMEGA matrix. This is primarily to feed into other
packages such as `mrgsolve`

.

`omega_matrix(r)`

- r
An nm object.

A `matrix`

object.

```
## requires NONMEM to be installed
if (FALSE) {
## matrix of initial estimates
m1 %>% omega_matrix()
## matrix of final estimates
m1 %>%
update_parameters() %>%
omega_matrix()
}
```