Removing duplicate code in pair_exp6_rx_kokkos

#include "force.h"

#include "neigh_list.h"

#include "math_const.h"

#include "math_special.h"

#include "math_special_kokkos.h"

#include "memory.h"

#include "error.h"

#include "modify.h"

using namespace LAMMPS_NS;

using namespace MathConst;

using namespace MathSpecial;

using namespace MathSpecialKokkos;

#define MAXLINE 1024

#define DELTA 4

  friend void pair_virial_fdotr_compute<PairExp6rxKokkos>(PairExp6rxKokkos*);

};

// optimized version of pow(x,n) with n being integer

// up to 10x faster than pow(x,y)

KOKKOS_INLINE_FUNCTION

static double powint(const double &x, const int n) {

  double yy,ww;

  if (x == 0.0) return 0.0;

  int nn = (n > 0) ? n : -n;

  ww = x;

  for (yy = 1.0; nn != 0; nn >>= 1, ww *=ww)

    if (nn & 1) yy *= ww;

  return (n > 0) ? yy : 1.0/yy;

};

}

#endif