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

src/compute_centro_atom.cpp

@@ -436,7 +436,7 @@ void ComputeCentroAtom::select2(int k, int n, double *arr, int *iarr)
 
 double ComputeCentroAtom::memory_usage()
 {
-  double bytes = nmax * sizeof(double);
+  double bytes = (double)nmax * sizeof(double);
   if (axes_flag) bytes += (double)size_peratom_cols*nmax * sizeof(double);
   return bytes;
 }