- 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.