Fix issue 8: Aspect set matroos is nu een lege dict

ddapioper.py

@@ -233,9 +233,14 @@ Returns the results part of the response.
     def aspectSet(self):
         """aspectSet()
 
-Returns the metadata of the aspectSet
+Returns the metadata of the aspectSet.
+
+Note: Return a empty dict if there is no aspectset (matroos)
 """
-        return(self.result()["observationType"]["aspectSet"]["aspects"])
+        observationType = self.result().get("observationType", {})
+        aspectSet       = observationType.get("aspectSet", {})
+        aspects         = aspectSet.get("aspects", {})
+        return(aspects)
 
     def location(self):
         """location()