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

[NickleDave]

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