[yaspgrid] rename *CoordinateContainer to *Coordinates
[yaspgrid,bugfix] we actually have to use references or pointers in the entity.

We require default constructability of the iterator, which implies in YaspGrid default
constructability of the entity. Thus we now us pointers and not references.
[iterator] add iterator tag to the leaf/level iterator
[test,bugfix] fix call to forward iterator test