add the usual LAMMPS copyright

/*

 fix_rhok.cpp

 A fix to add harmonic potential that bias |rho(k)|.

 The usage is as follows:

 fix [name] [groupID] rhok [nx] [ny] [nz] [K] [a]

 where k_i = (2 pi / L_i) * n_i

 Written by Ulf Pedersen and Patrick Varilly, 4 Feb 2010

 Tweaked for LAMMPS 15 Jan 2010 version by Ulf Pedersen, 19 Aug 2010

 Tweaked again March 4th   2012   by Ulf R. Pedersen, and

               September   2017   by Ulf R. Pedersen.

 */

/* ----------------------------------------------------------------------

   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator

   http://lammps.sandia.gov, Sandia National Laboratories

   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2003) Sandia Corporation.  Under the terms of Contract

   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains

   certain rights in this software.  This software is distributed under

   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.

------------------------------------------------------------------------- */

/* ----------------------------------------------------------------------

   Contributing author: Ulf R. Pedersen, ulf@urp.dk

------------------------------------------------------------------------- */

#include "fix_rhok.h"

#include "error.h"