[EntityIterator] Require a decrement method.

This is necessary to later implement bidirectional iterators.  I am not
implementing them yet, because I have no way to test them; there is no grid
currently that supports bidirectional iterators but not random-access
iterators.
[YaspGrid] Implement decrement() on the iterator implementation.

This is needed if we ever want to be able to implement bidirectional iterators
that are not random-access iterators at the same time.