datasetElementScopeKeys(datasetType, elementType) {
    return cachedKeys(`${datasetType}-${elementType}`,
      () => [[
        `datasets.${datasetType}.elements.${elementType}`,
        `datasets.${datasetType}`,
        `elements.${elementType}`,
        ''
      ]]);
  }
Returns the options scope keys for resolving element options that belong
to an dataset. These keys do not include the dataset itself, because it
is not under options.
@param {string} datasetType
@param {string} elementType
@return {string[][]}