Merge branch 'feat/service-layer' into feat/service-layer-adriaan

# Conflicts:
#	backend/src/controllers/classes.ts
#	backend/src/controllers/students.ts
#	backend/src/data/users/teacher-repository.ts
#	backend/src/interfaces/assignment.ts
#	backend/src/interfaces/teacher.ts
#	backend/src/routes/classes.ts
#	backend/src/services/assignments.ts
#	backend/src/services/class.ts
#	backend/src/services/students.ts
#	backend/src/util/translation-helper.ts

backend/src/interfaces/group.ts

@@ -0,0 +1,25 @@
+import { Group } from "../entities/assignments/group.entity.js";
+import { AssignmentDTO, mapToAssignmentDTO } from "./assignments.js";
+import { mapToStudentDTO, StudentDTO } from "./students.js";
+
+export interface GroupDTO {
+    assignment: number | AssignmentDTO,
+    groupNumber: number,
+    members: string[] | StudentDTO[],
+};
+
+export function mapToGroupDTO(group: Group): GroupDTO {
+    return {
+        assignment: mapToAssignmentDTO(group.assignment), // ERROR: , group.assignment.within),
+        groupNumber: group.groupNumber,
+        members: group.members.map(mapToStudentDTO),
+    }
+}
+
+export function mapToGroupDTOId(group: Group): GroupDTO {
+    return {
+        assignment: group.assignment.id,
+        groupNumber: group.groupNumber,
+        members: group.members.map(member => member.username),
+    }
+}