PointCloud crop with convex hull

I'd like to crop a pointcloud with a 3D Polygon e.g. from a convex hull or mesh.

It would be great if the [`PointCloud::Crop`](https://github.com/intel-isl/Open3D/blob/master/cpp/open3d/geometry/PointCloud.h#L174) would take a convex hull as input, additionally to a bounding box. I guess a mesh would be way more complicated and for most use cases a convex hull would be enough.

I was hoping the `o3d.visualization.SelectionPolygonVolume.crop_point_cloud(...)` could do that, but that seems to work only in 2D with fixed min/max values in the 3rd dimension. Eventhough the bounding_polygon is defined as Vector3d.

For my use case Trapezoids, Spheres and Cylinders are the most important shapes. So I'll try filtering over `Pointcloud::ComputePointCloudDistance` against the center points of the spheres or cylinders. As I understood the BoundingBox cannot be a Trapezoid. So i'll try to crop and combine the Trapezoid with differently aligned BoundingBoxes.