revised documentation and added a citation trigger in .cpp file

[Description:]

The local density (LD) potential is a new potential style that is, in some 

The local density (LD) potential is a mean-field manybody potential, and, in some 

sense,a generalization of embedded atom models (EAM). The name "local density 

potential" arises from the fact that it assigns an energy to an atom depending 

on the number of neighboring atoms of given type around it within a predefined 

#include "memory.h"

#include "error.h"

#include "domain.h"

#include "citeme.h"

using namespace LAMMPS_NS;

#define MAXLINE 1024

static const char cite_pair_local_density[] =

  "pair_style  local/density  command:\n\n"

  "@Article{Sanyal16,\n"

  " author =  {T.Sanyal and M.Scott Shell},\n"

  " title =   {Coarse-grained models using local-density potentials optimized with the relative entropy: Application to implicit solvation},\n"

  " journal = {J.~Chem.~Phys.},\n"

  " year =    2016,\n"

  " DOI = doi.org/10.1063/1.4958629"

  "}\n\n"

  "@Article{Sanyal18,\n"

  " author =  {T.Sanyal and M.Scott Shell},\n"

  " title =   {Transferable coarse-grained models of liquid-liquid equilibrium using local density potentials optimized with the relative entropy},\n"

  " journal = {J.~Phys.~Chem. B},\n"

  " year =    2018,\n"

  " DOI = doi.org/10.1021/acs.jpcb.7b12446"

  "}\n\n";

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

PairLocalDensity::PairLocalDensity(LAMMPS *lmp) : Pair(lmp)

  // set comm size needed by this pair

  comm_forward = 1;

  comm_reverse = 1;

  // cite publication

  if (lmp->citeme) lmp->citeme->add(cite_pair_local_density);

}

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