Remove cp.ndarray since cupy is optional

ndsl/quantity/quantity.py

@@ -231,7 +231,7 @@ def view(self) -> BoundedArrayView:
         return self._compute_domain_view
 
     @property
-    def data(self) -> Union[np.ndarray, cupy.ndarray]:
+    def data(self) -> np.ndarray:
         """the underlying array of data"""
         return self._data