[clang] Explicitly pass a floating point damping factor to Transfer::prolongate.

GCC warned about ambiguity between builtin operator*=(double&, int)
and FieldVector<double,i>::operator*=(int), but at least picked one
and just emitted a warning. Clang instead errored out.

To fix this this patch explicitly uses a floating point dampening
factor in the additive version of the AMG.

Fixes flyspray #1456 
https://dune-project.org/flyspray/index.php?do=details&task_id=1456.

dune/istl/paamg/amg.hh

@@ -874,7 +874,7 @@ namespace Dune
 
       for(typename Hierarchy<Domain,A>::Iterator coarseLhs = lhs--; coarseLhs != lhs_->finest(); coarseLhs = lhs--, --aggregates, --pinfo) {
         Transfer<typename OperatorHierarchy::AggregatesMap::AggregateDescriptor,Range,ParallelInformation>
-        ::prolongateVector(*(*aggregates), *coarseLhs, *lhs, 1, *pinfo);
+        ::prolongateVector(*(*aggregates), *coarseLhs, *lhs, 1.0, *pinfo);
       }
     }