Force object dtype to avoid unwanted iteration.

Something changed in NumPy 1.9 which means that np.array(my_list_of_Arrays) ends up iterating on the Array objects in the list using `__getitem__`. (This can take *ages*!) Using an explicit object dtype avoids this problem.

By adding `np.require(..., 'O')` to the `ArrayStack` constructor we avoid the need for user/test code to create the ndarray themselves and reduce the chance of the problem.