Tests and comments on metamodel selectors => DONE

client/src/app/metamodel/reducers/survey.reducer.spec.ts

...p/metamodel/selectors/attribute-distinct.selector.spec.ts

...nt/src/app/metamodel/selectors/attribute.selector.spec.ts

client/src/app/metamodel/selectors/column.selector.spec.ts

.../app/metamodel/selectors/criteria-family.selector.spec.ts

client/src/app/metamodel/selectors/database.selector.spec.ts

...c/app/metamodel/selectors/dataset-family.selector.spec.ts

client/src/app/metamodel/selectors/dataset.selector.spec.ts

client/src/app/metamodel/selectors/group.selector.spec.ts

client/src/app/metamodel/selectors/instance.selector.spec.ts

.../app/metamodel/selectors/output-category.selector.spec.ts

...rc/app/metamodel/selectors/output-family.selector.spec.ts

...c/app/metamodel/selectors/root-directory.selector.spec.ts

client/src/test-data.ts