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.cpp

@@ -58,7 +58,7 @@ Fix::Fix(LAMMPS *lmp, int /*narg*/, char **arg) :
 
   restart_global = restart_peratom = restart_file = 0;
   force_reneighbor = 0;
-  box_change_size = box_change_shape = box_change_domain = 0;
+  box_change = NO_BOX_CHANGE;
   thermo_energy = 0;
   thermo_virial = 0;
   rigid_flag = 0;