Refactor code for the updated features

[zhiltsov-max]

Summary

This PR polishes some recently added code to make it clearer and more correct. Required to be used in CVAT.

• Fix invalid polygon comparison in tests
• Improve docstring for the updated crop_covered_segments's ratio_tolerance parameter
• Added more functions for plain polygons, namely close_polygon and simplify_polygon

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