add device memtag

lib/walberla/experimental/memory/MemoryTags.hpp

@@ -38,6 +38,13 @@ struct unified : public _mem_tag
 {};
 inline unified unified_v;
 
+/**
+ * @brief Memory tag indicating GPU device memory.
+ */
+struct device : public _mem_tag
+{};
+inline device device_v;
+
 } // namespace memtag
 
 template< typename T >
@@ -81,6 +88,8 @@ struct SelectStandardAllocator< memtag::unified >
 
 } // namespace detail
 
+using MemTag = memtag::MemTag;
+
 /**
  * @brief Standard allocator implementation for a given memory tag and value type.
  *