Fix the creation of fresh attribute indices in DefineAttribute

This has already been fixed in the first overload that takes a `const T &value` argument, but is missing in the second overload that takes an array. Indices may currently be given out several times if attributes are deleted in between.

source/adios2/core/IO.tcc

@@ -161,12 +161,13 @@ Attribute<T> &IO::DefineAttribute(const std::string &name, const T *array,
     }
 
     auto &attributeMap = GetAttributeMap<T>();
-    const unsigned int size = static_cast<unsigned int>(attributeMap.size());
+    const unsigned int newIndex =
+        attributeMap.empty() ? 0 : attributeMap.rbegin()->first + 1;
 
-    auto itAttributePair =
-        attributeMap.emplace(size, Attribute<T>(globalName, array, elements));
+    auto itAttributePair = attributeMap.emplace(
+        newIndex, Attribute<T>(globalName, array, elements));
     m_Attributes.emplace(globalName,
-                         std::make_pair(helper::GetType<T>(), size));
+                         std::make_pair(helper::GetType<T>(), newIndex));
 
     return itAttributePair.first->second;
 }