Feature/data output

Improve VTKWriter

src/mesh/ItemToItemMatrix.hpp

@@ -62,6 +62,16 @@ class ItemToItemMatrix
   const ConnectivityMatrix& m_connectivity_matrix;
 
  public:
+  auto numberOfEntries() const
+  {
+    return m_connectivity_matrix.numEntries();
+  }
+
+  const auto& entries() const
+  {
+    return m_connectivity_matrix.entries();
+  }
+
   PASTIS_INLINE
   auto operator[](const SourceItemId& source_id) const
   {