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

src/compute_ke_atom.cpp

@@ -103,6 +103,6 @@ void ComputeKEAtom::compute_peratom()
 
 double ComputeKEAtom::memory_usage()
 {
-  double bytes = nmax * sizeof(double);
+  double bytes = (double)nmax * sizeof(double);
   return bytes;
 }