[Datumaro] Mean and std for dataset

[zhiltsov-max]

Motivation and context

When learning computer vision models, image mean and std computed channelwise are often used for nomalization to improve model quality. This PR adds a CLI command to compute them for a dataset.

datum import <path>
datum stats

How has this been tested?

Unit test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 5871

• 35 of 56 (62.5%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 65.769%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/components/operations.py	32	34	94.12%
datumaro/datumaro/cli/contexts/project/init.py	3	22	13.64%

Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/components/extractor.py	1	91.01%
cvat/apps/engine/media_extractors.py	5	75.88%

Totals
Change from base Build 5855:	-0.05%
Covered Lines:	10945
Relevant Lines:	16237

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💛 - Coveralls

[nmanovic]

LGTM