refactor: use SCThingTypes as type for type

src/core/protocol/routes/bulk/BulkRequest.ts

@@ -13,6 +13,7 @@
  * this program. If not, see <https://www.gnu.org/licenses/>.
  */
 import {SCAbstractRoute} from '../../../Route';
+import {SCThingType} from '../../../Thing';
 import {SCISO8601Date} from '../../../types/Time';
 
 /**
@@ -46,9 +47,8 @@ export interface SCBulkParameters {
   /**
    * Type of things that are indexed in this bulk.
    * 
-   * String is an element of the SCThingType enumeration.
    */
-  type: string;
+  type: SCThingType;
 }
 
 /**