Allow for setting dependencies in nested lists of tasks

**Description**

Allow for setting upstreams/downstreams of tasks contained in nested lists

**Use case / motivation**

Be able to do this:
```
t1 >> [[t2 >> t3 >> t4], [t5 >> t6 >> t7]] >> t8
```

Instead, currently it would be set up as:
```
t1 >> t2 >> t3 >> t4 >> t8
t1 >> t5 >> t6 >> t7 >> t8
```
