[Datumaro] Add rounding for coordinates

[zhiltsov-max]

Motivation and context

Coordinate rounding allows to reduce annotation file sizes and produce more comparable annotations.

• Added rounding for shape coordinates up to 2 digits
• Fixed polygon area computation

How has this been tested?

Unit 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 6763

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 69.116%

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Totals
Change from base Build 6761:	0.05%
Covered Lines:	11147
Relevant Lines:	15621

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💛 - Coveralls

[nmanovic]

@azhavoro , could you please look at the PR?

[azhavoro]

@zhiltsov-max Could you please update CHANGELOG.md?

[zhiltsov-max]

@azhavoro, it is updated, what do you mean?

[azhavoro]

@azhavoro, it is updated, what do you mean?

v1.1.0-beta was released yesterday, so I think your change notes should be moved to the [1.1.0] - Unreleased topic.

[zhiltsov-max]

Done