Use unsigned number type for VTK connectivity field

This avoids 'implicit narrowing' errors/warnings.

dune/gfe/cosseratvtkwriter.hh

@@ -178,8 +178,7 @@ public:
           else
             DUNE_THROW(Dune::IOError, "Unsupported element type '" << nE.first << "' found!");
         }
-        std::vector<int> connectivity(connectivitySize);
-
+        std::vector<unsigned int> connectivity(connectivitySize);
 
         size_t i=0;
         for (const auto& element : elements(gridView, Dune::Partitions::interior))

dune/gfe/vtkfile.hh

@@ -346,7 +346,7 @@ namespace Dune {
 
       std::vector<Dune::FieldVector<double,3> > points_;
 
-      std::vector<int> cellConnectivity_;
+      std::vector<unsigned int> cellConnectivity_;
 
       std::vector<int> cellOffsets_;