Fix world_shape default value

src/robofish/io/file.py

@@ -67,7 +67,7 @@ class File(h5py.File):
         mode: str = "r",
         *,  # PEP 3102
         world_size_cm: List[int] = None,
-        world_shape: str = None,
+        world_shape: str = "rectangle",
         validate: bool = False,
         validate_when_saving: bool = True,
         strict_validate: bool = False,