message: Remove MFEM_ATOMIC_ADD
diff: #define MFEM_THREAD_ID(k) threadIdx.k
  #define MFEM_THREAD_SIZE(k) blockDim.k
  #define MFEM_FOREACH_THREAD(i,k,N) for(int i=threadIdx.k; i<N; i+=blockDim.k)
  -#define MFEM_ATOMIC_ADD(x,y) atomicAdd(&x, y)
  #else
  #define MFEM_SHARED
  #define MFEM_SYNC_THREAD
  #define MFEM_THREAD_ID(k) 0
  #define MFEM_THREAD_SIZE(k) 1
  #define MFEM_FOREACH_THREAD(i,k,N) for(int i=0; i<N; i++)
  -#define MFEM_ATOMIC_ADD(x,y) x += y
  #endif
  namespace mfem
  