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

src/compute_pe_atom.cpp

@@ -200,6 +200,6 @@ void ComputePEAtom::unpack_reverse_comm(int n, int *list, double *buf)
 
 double ComputePEAtom::memory_usage()
 {
-  double bytes = nmax * sizeof(double);
+  double bytes = (double)nmax * sizeof(double);
   return bytes;
 }