Study: Add minor comment.

src/Model/Study.py

@@ -520,6 +520,9 @@ class Study(SQLModel):
             [self.scenarios, self._river]
         )
         logger.debug(cnt)
+        # HACK: 8 is the number of SQL request do and dont counted in
+        # submodel save_count (let's count sql execute into the
+        # '_save' method)
         return cnt + 8
 
     def close(self):