title: PCL::RegionGrowing on negative Indices
body: <p>I have a pointcloud from which I extracted a ground plane. Now I want to apply a region growing on all points except for the ground plane. How could I do that efficiently ?</p>
  <pre><code>pcl::SACSegmentationFromNormals &lt;pcl::PointXYZRGB, pcl::Normal&gt; ground_seg;
  ground_seg.setInputCloud(input_cloud);
  ground_seg.setInputNormals(input_normals);
  ground_seg.segment(*ground_indices, *coefficients);
  
  pcl::RegionGrowing&lt;pcl::PointXYZRGB, pcl::Normal&gt; region_growing;
  region_growing.setIndices(ground_indices); // In this case, region growing is applied on the plane indices
  </code></pre>