replace Fix::box_change_xxx variables with Fix::box_change bitmask and add check to Domain::init()

This allows a more specific tracking of whether multiple fixes are
modifying the same box parameter and error out in that case.

src/fix_balance.cpp

@@ -39,7 +39,7 @@ FixBalance::FixBalance(LAMMPS *lmp, int narg, char **arg) :
 {
   if (narg < 6) error->all(FLERR,"Illegal fix balance command");
 
-  box_change_domain = 1;
+  box_change = BOX_CHANGE_DOMAIN;
   scalar_flag = 1;
   extscalar = 0;
   vector_flag = 1;