[Infrastructure] Be more careful about field_type / real_type

This patch removes the assumption that the field_type and / or real_type
can always be implicitly converted to double and can be implicitly
constructed from an int (in constructs like field_type d = 0;).

This patch is required for compatibility with the FLOP counter in PDELab.

dune/istl/owneroverlapcopy.hh

@@ -412,7 +412,7 @@ namespace Dune {
           if (i->local().attribute()!=OwnerOverlapCopyAttributeSet::owner)
             mask[i->local().local()] = 0;
       }
-      result = 0;
+      result = T2(0.0);
 
       for (typename T1::size_type i=0; i<x.size(); i++)
         result += x[i]*(y[i])*mask[i];
@@ -439,10 +439,10 @@ namespace Dune {
           if (i->local().attribute()!=OwnerOverlapCopyAttributeSet::owner)
             mask[i->local().local()] = 0;
       }
-      double result = 0;
+      typename T1::field_type result = typename T1::field_type(0.0);
       for (typename T1::size_type i=0; i<x.size(); i++)
         result += x[i].two_norm2()*mask[i];
-      return sqrt(cc.sum(result));
+      return static_cast<double>(sqrt(cc.sum(result)));
     }
 
     typedef Dune::EnumItem<AttributeSet,OwnerOverlapCopyAttributeSet::copy> CopyFlags;

dune/istl/scalarproducts.hh

@@ -115,7 +115,7 @@ namespace Dune {
      */
     virtual double norm (const X& x)
     {
-      return x.two_norm();
+      return static_cast<double>(x.two_norm());
     }
   };
 

dune/istl/solver.hh

@@ -128,7 +128,7 @@ namespace Dune
     //! helper function for printing solver output
     template <class DataType>
     void printOutput(std::ostream& s,
-                     const double iter,
+                     const DataType& iter,
                      const DataType& norm,
                      const DataType& norm_old) const
     {
@@ -141,7 +141,7 @@ namespace Dune
     //! helper function for printing solver output
     template <class DataType>
     void printOutput(std::ostream& s,
-                     const double iter,
+                     const DataType& iter,
                      const DataType& norm) const
     {
       s << std::setw(iterationSpacing)  << iter << " ";