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/ItemArray.hpp

@@ -33,13 +33,13 @@ class ItemArray
 
   size_t m_size_of_arrays;
 
- public:
   // Allow const std:shared_ptr version to access our data
   friend ItemArray<std::add_const_t<DataType>, item_type, ConnectivitySharedPtr>;
 
   // Allow const std:weak_ptr version to access our data
   friend ItemArray<std::add_const_t<DataType>, item_type, ConnectivityWeakPtr>;
 
+ public:
   friend PUGS_INLINE ItemArray<std::remove_const_t<DataType>, item_type, ConnectivityPtr>
   copy(const ItemArray<DataType, item_type, ConnectivityPtr>& source)
   {