CLN/API: image_sequence subclasses FramesSequence; invert, gray remove

* In lieu of ``invert`` and ``gray``, ``ImageSequence`` accepts ``process_func``. Ultimately, perhaps the validation logic could move into the superclass.

* ``ImageSequence`` has no ``__next__`` method; you have to do

        i = iter(ImageSequence('dir'))
        next(i)

which is less confusing anyway

* ImageSequence accepts a dype arg, following TiffStack classes. Possibly some of this logic could move into the superclass as well.