Add Array<T>::data_type.

It is set by 'using data_type = T'

src/utils/Array.hpp

@@ -7,6 +7,9 @@
 template <typename DataType>
 class Array
 {
+ public:
+  using data_type = DataType;
+
  private:
   Kokkos::View<DataType*> m_values;