Allow custom subset names in YOLO

[zhiltsov-max]

Summary

Depends on #687
Closes #565

• Allowed custom subset names in YOLO import and export, except classes, names and backup
• Exporting a subset with a reserved name now leads to an export error

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