Add MARS (Import only)

[sizov-kirill]

Summary

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

[IRDonch]

There seems to be no verb here.

[sizov-kirill]

I fixed all comments and updated market1501 format for compatibility with mars format, there is changes that I made for market1501:

• Importer: made heuristic more unique
• Extractor: made query items as separate subset, because the original dataset don't suppose that query image relate with images from other subsets
• Load items: added attributes according original dataset description (I used track_id instead of sequence_id because these two attributes have the same meaning)
• Converter: implemented this strategy: if item has person_id attribute Converter try to save this item with original image naming convention, using default values for unknown attributes and incrementing frame_id for collisions.

I decided to keep both person_id attribute and Label for MARS format for compatibility with classification-task formats.
I also used the str type for person_id, for cases where users would like to mark some items with a special identifier.

[zhiltsov-max]

Looks good, please update Market docs.