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

src/compute_hexorder_atom.cpp

@@ -338,7 +338,7 @@ void ComputeHexOrderAtom::select2(int k, int n, double *arr, int *iarr)
 
 double ComputeHexOrderAtom::memory_usage()
 {
-  double bytes = ncol*nmax * sizeof(double);
+  double bytes = (double)ncol*nmax * sizeof(double);
   bytes += (double)maxneigh * sizeof(double);
   bytes += (double)maxneigh * sizeof(int);