Introduce API v1

[zhiltsov-max]

Summary

Resolves #133

Introduces comfortable library API. For external use, the recommended way to call Datumaro API is:

import datumaro as dm

• Added HLops module with some CLI command duplicates
• Added more function docs
• Some Dataset method parameters became kw-only
• Updated examples in docs
• Removed compatibility symbols for annotation classes in components.extractor

Remaining:

• Check how docs look like

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]

IMO, the fact that different values of this function select completely different implementations suggests that this function should be split into two, filter_items and filter_annotations.

[zhiltsov-max]

It was done so some time ago, but it wasn't very comfortable to use. I'll reconsider such option.