added 'using namespace' to find 'forEach'
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.