Scaling of `FiniteDifference` linear operator broken

[bwohlberg]

Scaling of the FiniteDifference linear operator broken; this simple example throws an exception:

from scico.linop import FiniteDifference

A = FiniteDifference(input_shape=(8, 8))
B = 2.0 * A

The problem is due to FiniteDifference being derived from VerticalStack, and the use of self.__class__ in the implementation of __mul__ in VerticalStack.