Add dataset level hint to schema that there should be at least one subject.

[rwblair]

And that subject should have a modality.

[rwblair]

Or generate error if fewer than 2 valid files

[effigies]

Or at least one that isn't a core file (dataset_description.json, CITATION.cff, README.*, CHANGES, LICENSE, genetic_info.json, participants.tsv, samples.tsv).

[effigies]

We have:

	[ERROR] There are no subject directories (labeled "sub-*") in the root of this dataset.
 (SUBJECT_FOLDERS)

		./dataset_description.json
			Evidence: schema.rules.rules.checks.dataset.SubjectFolders