Fix line matching in multimerge

[zhiltsov-max]

Summary

Closes #219

• Renamed smooth_line to approximate_line, added deprecation notice
• Added more descriptions and type annotations
• The function now always returns only a single array
• Updated implementation of the line comparison algorithm
• Fixed the problem when multimerge failed because of missing label categories
• Fixed area computation in case of self-intersections

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT