ENH: Add dataset sub-table to config file, remove other dataset/transform param keys

[NickleDave]

Core idea: add a dataset sub-table to top-level tabels [vak.train], [vak.eval], etc., in the config file

It will look like this:

[vak.train.dataset]
name = "BuiltInDataset"  # optional. Should exactly match class name
path = "~/path/to/data/on/my/computer"  # required
splits_path = "~/path/to/splits/I/made"  # optional
params = { window_size = 2000 }  # "optional" but required for some DataPipes

This assumes we have already done #685 and #345 (which I have--in progress now)

It allows the following:

• Replace train_transform_params + val_transform_params with the params option in the dataset table -- in practice we only ever use these parameters for specifying window_size; by enforcing the idea that these are parameters of the dataset we move towards ENH: Rename datasets to pipes, that have built-in transforms #724 where the built-in DataPipes have fixed transforms that they always use (and if you need more control then it's time to move to your own script)
• specifying a splits_path that is different from the default path -- this way if you want to try a different split with your dataset, you don't need to remake the entire dataset; splits are just in the splits file (edit: as in ENH: Make it possible to specify different splits for datasets #749)

[NickleDave]

Fixed by #750