Fix datumaro export with media

[zhiltsov-max]

Summary

Fixes #20

The problem required exporting from CVAT with images in Datumaro format, so users had problems with the format.

• Fix broken type inference for DatasetItem.media_as()
• Remove duplicated 'media' field in the datumaro format export
• Add PointCloud to the datumaro import namespace
• Add tests for the issue and datumaro dataset examples
• Fixed comparison for PointCloud objects

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2022 CVAT.ai Corporation
#
# SPDX-License-Identifier: MIT