ENH: Disconnected Signal(.data) iterators

Implements two iterators for iterating through the navigation axes of a Signal, and returning either a sub-Signal, or a slice of the data array. As it works with a copy of the AxesManager, it does not trigger any indexing connections on the original one, which can improve speed for those applications where those triggers are unnecessary. A good example for this is when the iteration triggers replots for each step, as this will many-double the time of processing.