use EnumMap instead of HashMap for ContributionTypes

src/main/java/org/heigit/ohsome/oshdb/tutorial/AdvandcedExample.java

 package org.heigit.ohsome.oshdb.tutorial;
 
 import java.util.Arrays;
-import java.util.HashMap;
+import java.util.EnumMap;
 import java.util.Map;
 import java.util.SortedMap;
 import org.heigit.ohsome.oshdb.OSHDBTimestamp;
@@ -62,7 +62,7 @@ public class AdvandcedExample {
     @Override
     public Map<ContributionType, Integer> get() {
       //create an empty result-map
-      Map<ContributionType, Integer> result = new HashMap<>();
+      Map<ContributionType, Integer> result = new EnumMap<>(ContributionType.class);
       //fill with 0 values
       Arrays.asList(ContributionType.values())
           .forEach(