Support within-Stack calls to ModelLayerBase._save_model to record intemediate outputs

There's a save_path kwarg in Stack.run that gets used after the last layer is called. But users should also be able to provide save_paths for intermediate layers. Suggest:

```
stack.set_save_path(layer_ind, save_path)
```

And it would be good to also be able to find layer indices based on the layer name:

```
stack.get_inds(layer_name)
```