Fix return type should be BoundingBox (#209)

The return type of these three methods is set to None, but it returns a BoundingBox.

src/data_morph/data/dataset.py

@@ -65,7 +65,7 @@ def __init__(
     def __repr__(self) -> str:
         return f'<{self.__class__.__name__} name={self.name} scaled={self._scaled}>'
 
-    def _derive_data_bounds(self) -> None:
+    def _derive_data_bounds(self) -> BoundingBox:
         """
         Derive bounds based on the data.
 
@@ -81,7 +81,7 @@ def _derive_data_bounds(self) -> None:
             ]
         )
 
-    def _derive_morphing_bounds(self) -> None:
+    def _derive_morphing_bounds(self) -> BoundingBox:
         """
         Derive morphing bounds based on the data.
 
@@ -99,7 +99,7 @@ def _derive_morphing_bounds(self) -> None:
         morph_bounds.adjust_bounds(x=x_offset, y=y_offset)
         return morph_bounds
 
-    def _derive_plotting_bounds(self) -> None:
+    def _derive_plotting_bounds(self) -> BoundingBox:
         """
         Derive plotting bounds based on the morphing bounds.