Fix a bug for 64-bit 2D Morton codes

We do (1 << 31). The problem is that MAX_INT (which is 2147483647) is
smaller than that, which leads to overflow.

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;