Add attributes in VOC format

[zhiltsov-max]

Motivation and context

Closes #1518

Allows any values in pose object attribute.
Allows to read and write annotation attributes in attributes element of VOC XML annotations.

Exporting is enabled by default and can be disabled with --allow-attributes=False CLI option of Datumaro export.

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 6157

• 30 of 37 (81.08%) changed or added relevant lines in 3 files are covered.
• 191 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.3%) to 65.141%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/plugins/voc_format/converter.py	14	15	93.33%
cvat/apps/dataset_manager/bindings.py	12	18	66.67%

Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/plugins/voc_format/converter.py	1	89.24%
datumaro/datumaro/util/annotation_tools.py	1	97.92%
cvat/apps/engine/media_extractors.py	6	75.88%
src/annotations-collection.js	29	63.37%
src/annotations-objects.js	154	62.15%

Totals
Change from base Build 6062:	-0.3%
Covered Lines:	11029
Relevant Lines:	16521

---

💛 - Coveralls

[azhavoro]

@zhiltsov-max 1 issue is here:
the value of checkbox attribute is dumped as True/False and imported as is, so that cannot be handled by UI properly. However for the CVAT format, the lower case is used: https://github.com/opencv/cvat/blob/develop/datumaro/datumaro/plugins/cvat_format/converter.py#L284

[nmanovic]

LGTM

[nmanovic]

@azhavoro , could you please look?

[zhiltsov-max]

I don't know if we should fail or just log if there is an incorrect attribute value.

[azhavoro]

@zhiltsov-max could you fix tests?

[nmanovic]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- datumaro/datumaro/plugins/voc_format/converter.py  4
- datumaro/datumaro/plugins/voc_format/extractor.py  2
         

See the complete overview on Codacy

[zhiltsov-max]

@azhavoro, done.