What exactly is the vertex density of a poisson reconstructed surface?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

[The tutorial](http://www.open3d.org/docs/latest/tutorial/Advanced/surface_reconstruction.html) on poisson surface reconstruction states the following about the returned vertex densities:

"The `create_from_point_cloud_poisson` function has a second densities return value that indicates for each vertex the density. A low density value means that the vertex is only supported by a low number of points from the input point cloud."

This gives a good intuition for the density values, but does not answer all my questions:

(1) How is density precisely defined / how is it computed?
(2) Is the density value deterministic?
(3) Is the density value comparable between multiple similarly dense pointclouds? E.g. if I have one pointcloud and split it in half, do I get the same density values for each triangle vertex as when I run the function for the whole pointcloud?
(4) Does it represent some exact number like 4 points from the input pointcloud formed this vertex?

[The original code](https://github.com/mkazhdan/PoissonRecon) mentions the `--density` flag: "Enabling this flag tells the reconstructor to output the estimated depth values of the iso-surface vertices." Is that the density value that the open3d function returns?

[The function docs](http://www.open3d.org/docs/latest/python_api/open3d.geometry.TriangleMesh.html#open3d.geometry.TriangleMesh.create_from_point_cloud_poisson) unfortunately don't mention anything about the density value.