Make sure that the relaxation factor for smoothers never is complex.

The note

/dune-istl/dune/istl/paamg/smoother.hh:130:26: note: the ABI of
passing structure with complex float member has changed in GCC 4.4

for DefaultConstructionArgs::getArgs got me thinking. I just could not
imagine why there should be comlex value involved. Turned out we
somehow assumed that for complex the relaxation factore would be
complex, too. IMHO a relaxation factor should scale and hence be real.
This MR make ssure that the relaxation factor is never complex.

Unforntunately, then a note for std::make_pair with a
complex in repartition.hh appeared:
/usr/include/c++/8/bits/stl_pair.h:524:5: note: the ABI of passing
structure with complex float member has changed in GCC 4.4
     make_pair(_T1&& __x, _T2&& __y)

and I just resorted to constructing Pair directly. Not sure whether
this is really needed but it should not hurt and ensures more silence.