Returns the dataset identifiers (e.g. "cad", "dfp") currently
covered. Discovered from the schemas installed under
inst/extdata/schemas/<group>/<dataset>/. When group is supplied,
restricts the result to datasets that live under that CKAN group;
otherwise returns every installed dataset across every group.
See also
Other discovery:
cvm_codelist(),
cvm_dataset_years(),
cvm_dictionary(),
cvm_groups(),
cvm_tables()