Fixes overwritten right hand side for FastAMG with one level.

Unfortunately, SuperLU always overwrites the right hand side with
the solution. Therefore we now copy it if there is only one level
in the matrix hierarchy of AMG.

dune/istl/paamg/fastamg.hh

@@ -415,7 +415,12 @@ namespace Dune
          pinfo->copyOwnerToAll(*lhs, *lhs);
          v=*lhs;
          }else{*/
-      mgc(v, d);
+      if(matrices_->maxlevels()==1){
+        // The coarse solver might modify the d!
+        Range b(d);
+        mgc(v, b);
+      }else
+        mgc(v, d);
       if(postSteps_==0||matrices_->maxlevels()==1)
         pinfo->copyOwnerToAll(v, v);
       //}