SV updates

If this is a 3-parent random mutation -- perhaps the default implementation could be 2 calls to a 2-parent random mutation.  
```
A, B, C = random.sample([A, B, C], 3)  # shuffle order of parents
mutate3(A, B, C) = mutate2(A, mutate2(B, C))
```

This might be better than doing nothing for parameters without this method implemented.