Added support for 3d annotations

[zapaishchykova]

• Added new function addObject3d
• Added to "annotation.xml" support for z-coordinates

[AndrewCarterUK]

Hey, thanks very much for contributing to the project!

There's a few things to go through before it would be safe to merge this:

1. The branch has a number of .idea files committed which shouldn't be there.

2. I'm not quite sure how this will behave with a 2D object? I should get some tests in place but it looks like it would still output 'z' co-ordinates which would break peoples existing code.

3. I've currently tried to stick pretty close to the PASCAL VOC format, I've had a look but I cannot find any official 3D extension to the format. Is this format one that is widely accepted? If not I would potentially be at risk of an official or more widely accepted extension being developed in a few years that this project became incompatible with.

Thanks again for the contribution, and sorry for being a bit reluctant at this point to merge. I just have to be quite careful with any changes as there are lots of derivative projects that depend on this library.

[zapaishchykova]

Hey! Thanks for the comments!

1. Sorry for the .idea files, I will remove them.

2. I looked thought several Yolo implementations and saw that they usually read the XML by tags. So theoretically, it will be ignored in the same way as tags , etc now. But you are right, maybe as a possible solution, 2 different templates should be there - both for 2d and 3d?

3. I could not find any implementation for a 3d Pascal Voc either, that's why I thought this would be a nice addition to the existing library. Although nowadays there are not that many 3d object detection architectures my feeling is that they still have the use-cases.

What do you think, is it better to create a separate library for 3d support rather than extend the existing one?