changing limiter from minmod to koren

#if DIMENSIONS == 3

inline double kernels::finitevolumes::musclhancock::c::minmod(double a, double b) {

inline double kernels::finitevolumes::musclhancock::c::minmod(double b, double a) {

  assert(std::isfinite(a));

  assert(std::isfinite(b));

//koren limiter

  if(std::abs(a) < 1.e-12) { 

    return 0.0;

  } else {

    double r=b/a;

    double phi=std::max(0.0, std::min(2.0*r , std::min((1.+2.*r)/3.,2.0)));

    return phi*a;

  }

/*

  if (a * b < 0.0) {  // sign is different (alternative: std::signbit and xor)

    return 0.0;

  } else {

      return b;

    }

  }

*/

}

// TODO(Dominic): Work in progress