Use only one iteration step for BlockSize==1

This is already exact, so there is no need to do 20 steps

[[Imported from SVN: r5919]]

dune/solvers/iterationsteps/projectedblockgsstep.cc

@@ -55,7 +55,7 @@ void ProjectedBlockGSStep<OperatorType, DiscFuncType>::iterate()
             // Initial correction
             v = 0;
 
-            for (int j=0; j<20; j++) {
+            for (int j=0; j< ((BlockSize==1) ? 1 : 20); j++) {
 
                 for (int k=0; k<BlockSize; k++) {