Add SubArray class

This allow to manipulate contiguous sub parts of arrays in the same way as arrays themselves

Note that for performances reasons, SubArray cannot be Array

src/mesh/IConnectivity.hpp

@@ -13,6 +13,9 @@ class IConnectivity : public std::enable_shared_from_this<IConnectivity>
   template <typename DataType, typename ItemOfItem, typename ConnectivityPtr>
   friend class SubItemValuePerItem;
 
+  template <typename DataType, typename ItemOfItem, typename ConnectivityPtr>
+  friend class SubItemArrayPerItem;
+
   virtual const ConnectivityMatrix& _getMatrix(const ItemType& item_type_0, const ItemType& item_type_1) const = 0;
 
  public: