Dataset meta file for some formats (Cityscapes, ImageNet Txt, Pascal VOC and WiderFace)

[yasakova-anastasia]

Summary

This is the first PR. It contains changes in the Cityscapes, ImageNet Txt, Pascal VOC and WiderFace formats.

• Added save_meta_file, is_meta_file, has_meta_file and parse_meta_file functions
• Added a parameter in the converter to save the dataset meta file

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

[zhiltsov-max]

Ok, please fix the remaining topics and it can be merged.