Improve stats computation

[zhiltsov-max]

Summary

• Fixes warnings described in datum stats and transform with resize failed to run on part of PascalVOC dataset #607 in stats and multimerge for lines
• Adds more filtering options to the stats command
• Improved performance of image stat computations (~2 times)
• Image uniqueness stats moved to image stats category
• Update docs

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

[yasakova-anastasia]

I think it looks good.