more typecasts to double to avoid 32-bit int overflows when computing memory usage

src/compute_cluster_atom.cpp

@@ -267,6 +267,6 @@ void ComputeClusterAtom::unpack_forward_comm(int n, int first, double *buf)
 
 double ComputeClusterAtom::memory_usage()
 {
-  double bytes = nmax * sizeof(double);
+  double bytes = (double)nmax * sizeof(double);
   return bytes;
 }