added constraints to frontend

...r/masterthesis/Testgenerator/bpmn/data/VariableValue.java

...thesis/Testgenerator/rest/dto/FlowElementEnhancedDTO.java

...s/Testgenerator/rest/service/change/ChangeController.java

.../Testgenerator/rest/service/converters/TestConverter.java

...hesis/Testgenerator/rest/service/test/TestController.java

...erthesis/Testgenerator/rest/service/test/TestService.java

...mponents/new-external-mock/new-external-mock.component.ts

...generator-web/src/app/components/test/test.component.html

...stgenerator-web/src/app/components/test/test.component.ts

...enerator-web/src/app/models/bpmn-flow-enhanced-element.ts

...or-web/testgenerator-web/src/app/models/variable-value.ts