Hillslope elevation profile

[changliao1025]

Currently, the hillslope feature requires at least enough elevation value:

if (vElevation_left.size() > 11)
{
  aElevation_profile_left[0] = *std::min_element(vElevation_left.begin(), vElevation_left.end());
  aElevation_profile_left[10] = *std::max_element(vElevation_left.begin(), vElevation_left.end());
  for (int i = 1; i < nElevation_profile; i++)
  {
    p = i * 10.0;
    aElevation_profile_left[i] = data::percentile(vElevation_left, p);
  }
  // now calculate the hillslope slope using the elevation profile
  dSlope_hillslope_left = (aElevation_profile_left[10] - aElevation_profile_left[0]) / dLength_hillslope_left;
}

[changliao1025]

This data::percentile can be relaxed to allow interpolation even if less than 11 values are provided in future version.