ENH: add dataclasses to represents results directories, e.g. `TrainResults` and `LearncurveResults`

to make it easier to work with the results programmatically.

Will represent the structure of a `results_dir_{timestamp}` created by one run of `vak learncurve` or `vak train`

something like
```
class LearncurveResults
    ckpt_paths: list
    dataset_paths: list
```