Fix a warning in Morton codes

"integer conversion resulted in a change of sign" for (1 << 31).

src/details/ArborX_DetailsMortonCode.hpp

@@ -114,7 +114,7 @@ unsigned long long morton64(float x, float y)
 {
   // The interval [0,1] is subdivided into 2,147,483,648 bins (in each
   // direction).
-  constexpr unsigned N = (1 << 31);
+  constexpr unsigned N = (1u << 31);
 
   using KokkosExt::max;
   using KokkosExt::min;