AMGStep: always setup dune-istl's AMG in `preprocess`

The `amg_` member initialized in the constructor is never used as
`preprocess()` unconditionally resets it.

We also create `fop_` only in `preprocess()` to have only one place
when it is initialized.

dune/solvers/iterationsteps/amgstep.hh

@@ -47,10 +47,6 @@ public:
              const Criterion& coarseningCriterion)
     {
         setProblem(*stiffnessMatrix, x, rhs);
-
-        fop_ = std::unique_ptr<Operator>(new Operator(*stiffnessMatrix));
-        amg_ = std::unique_ptr<AMG>(new AMG(*fop_, coarseningCriterion, smootherArgs));
-        amg_->pre(*this->x_, residual_);
     }
 
     /** \brief Initialize the iteration step but don't actually build the matrix hierarchy yet */
@@ -68,7 +64,6 @@ public:
     {
         LinearIterationStep<MatrixType, VectorType>::setProblem(stiffnessMatrix, x, rhs);
 
-        fop_ = std::unique_ptr<Operator>(new Operator(stiffnessMatrix));
         residual_ = rhs;
     }
 
@@ -108,6 +103,7 @@ template <class MatrixType, class VectorType>
 void AMGStep<MatrixType,VectorType>::preprocess()
 {
     setupCoarseningCriterion();
+    fop_ = std::unique_ptr<Operator>(new Operator(*this->getMatrix()));
     amg_ = std::unique_ptr<AMG>(new AMG(*fop_, coarseningCriterion_, smootherArgs_));
     amg_->pre(*this->x_, residual_);
 }