allow accessing a ListMonitor as a flat array when using API's without section info

CoreStore/Observing/ListMonitor.swift

@@ -82,7 +82,7 @@ public final class ListMonitor<T: NSManagedObject> {
      */
     public subscript(index: Int) -> T {
         
-        return self[0, index]
+        return self.objectsInAllSections()[index]
     }
     
     /**
@@ -93,7 +93,12 @@ public final class ListMonitor<T: NSManagedObject> {
      */
     public subscript(safeIndex index: Int) -> T? {
         
-        return self[safeSectionIndex: 0, safeItemIndex: index]
+        let objects = self.objectsInAllSections()
+        guard objects.indices.contains(index) else {
+            
+            return nil
+        }
+        return objects[index]
     }
     
     /**