Added interface for anchor objects. (#43)

Author: Kevin Hellemun

BunqSdk/Model/Core/AnchorObjectInterface.cs

@@ -0,0 +1,9 @@
+namespace Bunq.Sdk.Model.Core
+{
+    public interface IAnchorObjectInterface
+    {
+        bool AreAllFieldNull();
+
+        BunqModel GetReferencedObject();
+    }
+}