Merge branch 'feature/fix-bitsetvectortest-libc++' into 'master'

Fix compilation of bitsetvectortest with libc++

The new version tests that a BitSetVectorConstReference can be converted
to a bitset, which makes sense.

The old version tested if a std::vector<bool>::const_reference could be
converted to a bitset, which should not be assumed or tested, even
though it works with libstdc++, apparently

See also #18.

See merge request !59

dune/common/test/bitsetvectortest.cc

@@ -20,7 +20,7 @@ struct ConstReferenceOp
   typedef typename BBF::const_reference const_reference;
 
   void operator()(const_reference t){
-    DUNE_UNUSED bitset x = t[0];
+    DUNE_UNUSED bitset x = t;
   }
 };