added tool to convert existing NSManagedObjectModels to the new CoreStoreSchema

Sources/Setup/DataStack.swift

@@ -96,6 +96,14 @@ public final class DataStack: Equatable {
         return self.schemaHistory.currentModelVersion
     }
     
+    /**
+     Returns the `DataStack`'s model schema.
+     */
+    public var modelSchema: DynamicSchema {
+        
+        return self.schemaHistory.schemaByVersion[self.schemaHistory.currentModelVersion]!
+    }
+    
     /**
      Returns the entity name-to-class type mapping from the `DataStack`'s model.
      */