- the DUNE_UNUSED macro is not a function, this lead to
some remaining warnings about unused variables

with 3.8 we were seeing kinda weird constructs, i.e. in for loops

This should always be the norm as it shows problems with missing
includes early.

-#include <dune/stuff/common/logging.hh>
+#include <dune/stuff/common/timedlogging.hh>
is encouraged, but not mandatory yet

I left the {f,o,s}stream includes in place although they are not needed
as far as I can tell. Remove them too, @renemilk?

mpihelper.hh is moved in dune-common >= 2.3

it's much better to inform the user about missing Logger initialization

refs #191