[Datumaro] Add dataset examples

[zhiltsov-max]

Motivation and context

Adds examples for datasets in tests. The corresponding tests depend on #1463 and added there.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 5446

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 65.666%

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Totals
Change from base Build 5409:	0.02%
Covered Lines:	10838
Relevant Lines:	16101

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💛 - Coveralls

[azhavoro]

@zhiltsov-max Are these assets used for tests?

[zhiltsov-max]

@azhavoro, the corresponding tests are added in #1463.