Permit multiple instances, and None for loading data using `EEGDashDataset`

[neuromechanist]

The current behavior for EEGDashdatasets is:

    dataset = EEGDashDataset({
        "dataset": dataset_name, 
        "subject": subject,
        "task": task,
        "run": run,
        })

None of the subject, task, run, or in general (description_fields) can be a list or of None type. They only can be of a specific value.
This way, if you need to pull two subjects, runs or tasks, you need to do it in a for loop.

Permitting the field values accept list or if passed as None be ignored will make much cleaner implementations.

[arnodelorme]

Yes, it would be a good improvement.