replace some numeric constants in Atom and AtomVec classes with enumerators

src/fix_read_restart.cpp

@@ -32,7 +32,7 @@ FixReadRestart::FixReadRestart(LAMMPS *lmp, int narg, char **arg) :
   // register with Atom class
 
   grow_arrays(atom->nmax);
-  atom->add_callback(0);
+  atom->add_callback(Atom::GROW);
 
   // extra = copy of atom->extra
 
@@ -54,7 +54,7 @@ FixReadRestart::~FixReadRestart()
 {
   // unregister callback to this fix from Atom class
 
-  atom->delete_callback(id,0);
+  atom->delete_callback(id,Atom::GROW);
 
   // delete locally stored arrays