added 'using namespace' inside generic lambdas to workaround an intel compiler bug

This is related to issue 31 in dune-istl.

Due to an intel compiler bug (it's probably a bug since all other compilers I've tested do not have this issue and it should not be related to compiler flags) a small workaround is necessary in nested functions calls involving generic lambda, see also my stackoverflow question. Especially this occurs in nested Hybrid::forEach calls due to the usage of the subnamespace Hybrid that is not found inside of the lambdas.

The second change removes a warning thrown by intel compilers on unnecessary const on POD return types.