[EntityIterator] Fix the code in the placeholder comment for operator[].
[EntityIterator] Fix the semantics of the implementation method distanceTo().

It now has the same sense as for the iterator facades.

Thanks to Markus Blatt for noticing this.
[YaspGrid] Fix the semantics of the method distanceTo() in the entity iterator
implementation.

It now has the same sense as for the iterator facades.

Thanks to Markus Blatt for noticing this.