Auto add value field if aggregations are requested.

src/main/java/ch/psi/daq/queryrest/controller/validator/QueryValidator.java

@@ -79,6 +79,11 @@ public class QueryValidator implements Validator {
          if (query.getAggregation().getAggregations() == null || query.getAggregation().getAggregations().isEmpty()) {
             query.getAggregation().setAggregations(new ArrayList<>(defaultResponseAggregations));
          }
+         
+         if (!query.getFields().contains(QueryField.value)) {
+            // without this field, json will not contain Stats (as 
+            query.addField(QueryField.value);
+         }
       }
    }
 }