Simplify using smoother as coarse solver

Is no base solver is provided, the MutliGridStep does now
do pre- and postsmoothing on the coarse level as it does
on any other level.

dune/solvers/iterationsteps/multigridstep.cc

@@ -146,6 +146,8 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::preprocess()
     //   Set up base solver
     // /////////////////////////////////////////////
 
+    if (basesolver_)
+    {
         if (this->basesolver_ == NULL)
             DUNE_THROW(SolverError, "You have not provided a base solver!");
 
@@ -191,6 +193,7 @@ else {
                        << " as a base solver in a MultigridStep!");
         }
     }
+}
 
 
 template<class MatrixType, class VectorType, class BitVectorType>
@@ -226,7 +229,7 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::iterate()
     std::vector<VectorType>& rhs = this->rhsHierarchy_;
 
     // Solve directly if we're looking at the coarse problem
-    if (level == 0) {
+    if ((level == 0) and (basesolver_)) {
         basesolver_->solve();
         return;
     }
@@ -239,6 +242,8 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::iterate()
     for (int i=0; i<this->nu1_; i++)
         presmoother_[level]->iterate();
 
+    if (level>0)
+    {
         // /////////////////////////
         // Restriction
 
@@ -271,6 +276,7 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::iterate()
         VectorType tmp;
         this->mgTransfer_[level-1]->prolong(*x[level-1], tmp);
         *x[level] += tmp;
+    }
 
     // Postsmoothing
     postsmoother_[level]->setProblem(*(mat[level]), *x[level], rhs[level]);