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.