Add argument to children() to yield names of the modules as well

Currently, in order to selectively iterate through the modules in a network, we need to do something like
```python
for name, module in model._modules.items():
    if name in KEEP_LIST:
        ...
```
It would be good to add an option to `.children()` (and eventually to `.modules()` as well) to yield the `name` of the modules as well. Maybe something like `.children(named=True)`?