refactor: refactor SCThingTypes from union-type to enum. SCThingType-type-attributes are now string-type-attributes.

src/core/things/Semester.ts

@@ -13,12 +13,7 @@
  * this program. If not, see <https://www.gnu.org/licenses/>.
  */
 import {SCAcademicTermWithoutReferences} from '../base/AcademicTerm';
-import {SCThingMeta} from '../Thing';
-
-/**
- * Type of a semester
- */
-export type SCSemesterType = 'semester';
+import {SCThingMeta, SCThingType} from '../Thing';
 
 /**
  * A semester without references
@@ -34,13 +29,17 @@ export interface SCSemesterWithoutReferences extends SCAcademicTermWithoutRefere
   /**
    * Type of the semester
    */
-  type: SCSemesterType;
+  type: SCThingType.Semester;
 }
 
 /**
  * A semester
  */
 export interface SCSemester extends SCSemesterWithoutReferences {
+  /**
+   * Type of the semester
+   */
+  type: SCThingType.Semester;
 }
 
 /**