Load / Save Viewpoint for Orthographic Camera

Hi, thanks for all the awesome work on this library.

I'm looking for a way to save and load the viewpoint of the **orthographic camera** in the visualizer using the Python bindings. I am aware of this https://github.com/intel-isl/Open3D/pull/597 which resulted in this [example](https://github.com/intel-isl/Open3D/blob/master/examples/Python/Advanced/load_save_viewpoint.py), but unless I am using it wrongly I believe that it only works for non-orthographic cameras. That is, when the FOV is set to 5, saving and loading produces a different view.

The closest lead I've found so far is in the visualizer commands [Ctrl+C and Ctrl+V for storing and loading the view](https://github.com/intel-isl/Open3D/blob/edb5272e80a9996c843c1883cba301bacb0d8fb6/docs/tutorial/Basic/visualization.rst), which works perfectly for orthographic and any view. This is exactly the behavior I want but I have not found a way to achieve this programmatically in Python, and my guess is that this relies on the C++ `ConvertFromViewParameters` which is not exposed yet in the Python API.

https://github.com/intel-isl/Open3D/blob/995b646375121e348ff814b138a924bfac0f047e/src/Visualization/Visualizer/ViewControl.cpp#L96

This would be a great help to my projects, please help or let me know if I am missing something.

@syncle