Refactor: Only prepare unfiltered Features

CityDoctorParent/CityDoctorValidation/src/main/java/de/hft/stuttgart/citydoctor2/check/Checker.java

@@ -756,11 +756,9 @@ public class Checker {
                 }
                 if(Thread.interrupted()){
                     Thread.currentThread().interrupt();
-                    return null;
+                    return id;
                 }
-                co.prepareForChecking();
                 executeChecksForCityObject(co);
-                co.clearMetaInformation();
                 cache.put(co);
                 checkedCount.incrementAndGet();
                 if (l!=null){
@@ -833,7 +831,9 @@ public class Checker {
         if (!filterObject(co)) {
             return;
         }
+        co.prepareForChecking();
         executeChecksForCheckable(co);
+        co.clearMetaInformation();
     }
 
     /**