Support for planar subdivisions in image processing

This is more of a feature request than anything else. I'm working on some image manipulations to implement something along the lines of [this faceswapping tutorial](https://pysource.com/2019/04/04/face-swapping-opencv-with-python-part-1/) and will need access to delaunay triangulation methods.

I've identified the need to support [this Subdiv2D class](https://docs.opencv.org/3.4/df/dbf/classcv_1_1Subdiv2D.html) as well as the need to add support for vectors of 6 dimensions.

I'm interested in actively contributing this back, but I am feeling in over my head so I might not get this completed. If I don't, this issue should be here for future users to track progress.

Start with the OpenCV Subdiv2D documentation linked in the issue and inspect how the existing bindings represent vectors and other OpenCV types. Determine the entry points and tests for adding Subdiv2D and six-dimensional vectors; done means both are exposed and covered by the project's relevant tests.

Written by the indexing model from the issue text.