[Datumaro] Merge with different categories

[zhiltsov-max]

Motivation and context

This capability allows to merge projects with different label sets.

How has this been tested?

Unit tests, manual tests

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 7276

• 126 of 139 (90.65%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 70.802%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/util/annotation_util.py	0	1	0.0%
datumaro/datumaro/components/operations.py	117	129	90.7%

Totals
Change from base Build 7274:	0.1%
Covered Lines:	12500
Relevant Lines:	17263

---

💛 - Coveralls