Default implement getSol()

This is the first step towards getting rid of the infamous
getSol() method which has several problems: It returns
a copy, it's non-const, it allows that iteration steps
bypass the x_ member which is dedicated to storing the
iterate.

Having the default implementation allows to remove the
implementation of this method from most interation
steps.

dune/solvers/iterationsteps/iterationstep.hh

@@ -54,7 +54,10 @@ class IterationStep : virtual public NumProc, public CanIgnore<BitVectorType>
         }
 
         //! Return solution object
-        virtual VectorType getSol() = 0;
+        virtual VectorType getSol()
+        {
+            return *getIterate();
+        }
 
         /** \brief Checks whether all relevant member variables are set
          * \exception SolverError if the iteration step is not set up properly