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

src/compute_cna_atom.cpp

@@ -355,7 +355,7 @@ void ComputeCNAAtom::compute_peratom()
 
 double ComputeCNAAtom::memory_usage()
 {
-  double bytes = nmax * sizeof(int);
+  double bytes = (double)nmax * sizeof(int);
   bytes += (double)nmax * MAXNEAR * sizeof(int);
   bytes += (double)nmax * sizeof(double);
   return bytes;