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

src/fix_respa.cpp

@@ -74,7 +74,7 @@ int FixRespa::setmask()
 
 double FixRespa::memory_usage()
 {
-  double bytes = atom->nmax*nlevels*3 * sizeof(double);
+  double bytes = (double)atom->nmax*nlevels*3 * sizeof(double);
   if (store_torque) bytes += (double)atom->nmax*nlevels*3 * sizeof(double);
   return bytes;
 }