add deprecation warning to fix langevin and document it

doc/src/Commands_removed.rst

@@ -12,6 +12,17 @@ stop LAMMPS and print a suitable error message in most cases, when a
 style/command is used that has been removed or will replace the command
 with the direct alternative (if available) and print a warning.
 
+GJF formulation in fix langevin
+-------------------------------
+
+.. deprecated:: TBD
+
+The *gjf* keyword in fix langevin is deprecated and will be removed
+soon.  The GJF functionality has been moved to its own fix style
+:doc:`fix gjf <fix_gjf>` and it is strongly recommended to use that
+fix instead.
+
+
 LAMMPS shell
 ------------
 

doc/src/fix_langevin.rst

@@ -241,6 +241,13 @@ to zero by subtracting off an equal part of it from each atom in the
 group.  As a result, the center-of-mass of a system with zero initial
 momentum will not drift over time.
 
+.. deprecated:: TDB
+
+The *gjf* keyword in fix langevin is deprecated and will be removed
+soon.  The GJF functionality has been moved to its own fix style
+:doc:`fix gjf <fix_gjf>` and it is strongly recommended to use that
+fix instead.
+
 The keyword *gjf* can be used to run the :ref:`Gronbech-Jensen/Farago
 <Gronbech-Jensen>` time-discretization of the Langevin model.  As
 described in the papers cited below, the purpose of this method is to

src/fix_langevin.cpp

@@ -170,6 +170,9 @@ FixLangevin::FixLangevin(LAMMPS *lmp, int narg, char **arg) :
   // no need to set peratom_flag, b/c data is for internal use only
 
   if (gjfflag) {
+    if (comm->me == 0)
+      error->warning(FLERR, "The GJF formulation in fix {} is deprecated and will be removed soon. "
+                     "\nPlease use fix gjf instead: https://docs.lammps.org/fix_gjf.html", style);
     FixLangevin::grow_arrays(atom->nmax);
     atom->add_callback(Atom::GROW);