now the if clause to get the twist and not whether the item is the same

an implementation for a levelintersectioniterator for conforming refinement, that is restricted to the macro level

Because the CommDataHandleIF exports the DataType, it is indeed possible
to check whether a type T is derived from
(*)    CommDataHandle< T, typenanme T::DataType >.
We use this information to disable the generic method loadBalance( LBWeights )
in this case.

Note: If someone passes a type U derived from a type T derived from (*),
there is no way of checking this. In this case, the wrong method will be
used, hopefully resulting in a compile time error.

Note: As a side effect, the iterator test triggers an error in the edge
iterator of a 2d grid. Here, not all entities are visited.

grids seems not to work.

The only load-balancing algorithm useable without Zoltan is based on a
space filling curve. Internally, however, this algorithm assumes that
the macro elements are already sorted along such a curve. Originally,
this was ensured by sorting them along the Hilbert curve provided by
Zoltan.  To make dune-alugrid useful without Zoltan, we still need a
space filling curve implementation of our own.

This patch makes dune-alugrid more self-contained by providing a simple
implementation of a Z curve. It is used only when Zoltan is not found,
so experienced users will not notice the difference.