Adding missing rank to xtensor_adaptor

Author: tdegeus

include/xtensor/xtensor.hpp

@@ -225,6 +225,7 @@ namespace xt
         using backstrides_type = typename base_type::backstrides_type;
         using temporary_type = typename semantic_base::temporary_type;
         using expression_tag = Tag;
+        constexpr static std::size_t rank = N;
 
         xtensor_adaptor(storage_type&& storage);
         xtensor_adaptor(const storage_type& storage);

include/xtensor/xutils.hpp

@@ -875,7 +875,7 @@ namespace xt
     template <class E>
     struct get_rank<E, decltype((void)E::rank, void())>
     {
-        constexpr static std::size_t value= E::rank;
+        constexpr static std::size_t value = E::rank;
     };
 
     /******************