Avoid compiler warnings, mainly with DUNE_UNUSED.

dune/common/densevector.hh

dune/common/test/bitsetvectortest.cc

dune/common/test/deprtuplestest.cc

dune/common/test/dynmatrixtest.cc

dune/common/test/dynvectortest.cc

dune/common/test/fmatrixtest.cc

dune/common/test/fvectortest.cc

dune/common/test/genericiterator_compile_fail.cc

dune/common/test/iteratortest.hh

dune/common/test/parametertreetest.cc

dune/common/test/poolallocatortest.cc

dune/common/test/tuplestest.cc

dune/common/test/utilitytest.cc