[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.