message: BUGFIX: use cub sort when using CUDA_EXEC async
  Fix template specialization of custom_sort() routine, that
  is used internally by the BVH to sort the supplied AABBs
  according to their morton index, to dispacth to the CUB
  sort, when the specified execution policy is CUDA_EXEC
  async.
  Resolves
diff: @@ -403,13 +403,13 @@ void custom_sort( ExecSpace, uint32*& mcodes, int32 size, int32* iter )
  //------------------------------------------------------------------------------
  #if defined(AXOM_USE_CUDA) && defined(AXOM_USE_RAJA) && \
  defined(RAJA_ENABLE_CUDA) && defined(AXOM_USE_CUB)
  -template < int BLOCK_SIZE >
  -void custom_sort( axom::CUDA_EXEC< BLOCK_SIZE >,
  +template < int BLOCK_SIZE, axom::ExecutionMode EXEC_MODE >
  +void custom_sort( axom::CUDA_EXEC< BLOCK_SIZE, EXEC_MODE >,
  uint32*& mcodes, int32 size, int32* iter )
  {
  AXOM_PERF_MARK_FUNCTION( "custom_sort" );
  - using ExecSpace = typename axom::CUDA_EXEC< BLOCK_SIZE >;
  + using ExecSpace = typename axom::CUDA_EXEC< BLOCK_SIZE, EXEC_MODE >;
  array_counting< ExecSpace >(iter, size, 0, 1);
  AXOM_PERF_MARK_SECTION( "gpu_cub_sort",
  