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/SubItemValuePerItem.hpp

@@ -109,6 +109,17 @@ class SubItemValuePerItem
     return m_connectivity_ptr.use_count() != 0;
   }
 
+  PUGS_INLINE
+  std::shared_ptr<const IConnectivity>
+  connectivity_ptr() const noexcept
+  {
+    if constexpr (std::is_same_v<ConnectivityPtr, ConnectivitySharedPtr>) {
+      return m_connectivity_ptr;
+    } else {
+      return m_connectivity_ptr.lock();
+    }
+  }
+
   template <typename IndexType, typename SubIndexType>
   PUGS_FORCEINLINE DataType&
   operator()(IndexType item_id, SubIndexType i) const noexcept(NO_ASSERT)
@@ -128,7 +139,7 @@ class SubItemValuePerItem
   {
     static_assert(std::is_integral_v<ArrayIndexType>, "index must be an integral type");
     Assert(this->isBuilt());
-    Assert(i < m_values.size());
+    Assert(static_cast<size_t>(i) < m_values.size());
     return m_values[i];
   }
 
@@ -223,7 +234,6 @@ class SubItemValuePerItem
   {
     static_assert(not std::is_const_v<DataType>, "Cannot allocate SubItemValuePerItem of const data: only "
                                                  "view is supported");
-    ;
 
     ConnectivityMatrix connectivity_matrix = connectivity._getMatrix(item_type, sub_item_type);