Dataset config: source placeholders, cell strain format fix, ground_truth_status / method_type fix; schema improvements

[daniel-ji]

• [x] Add sources placeholders for 10003, 10006, 10426-435.yaml (to pass validation for now)
• [x] Fixed format of datasets.cell_strain.id (NCBI:txid2325 -> NCBITaxon:2325) 10008.yaml
• [x] Made method_type automated -> hybrid since ground_truth_status is true for 10006.yaml (can't have automated and true co-exist)
• [x] Add minimum_cardinality field via schema.py for all required multivalued fields (so that a required list value cannot be an empty list)
• [x] Also a small fix of DatestampedEntity -> DateStampedEntity (a small change, doesn't break anything)