version the expression was put into a sizeof which avoids the not used
warning but does require that the expression is well defined. In many
places in alu that is not the case because the not used warning is removed
at the place it occurs.

Before the program aborts after an assert fails, it is now
possible to attach a debugger (if the correct defines are set).

- instead of #ifndef NDEBUG
  now use #ifdef ALUGRIDDEBUG
- istead of assert( cond );
  now use alugrid_assert ( cond );
  (from dune/alugrid/common/alugrid_assert.hh)
So instead of having to define NDEBUG to avoid debug code being executed
(improves performance by 60%) one always avoids debug code as long as
ALUGRIDDEBUG has not been define.