Move merge policies to datumaro/components/merge directory

[vinnamkim]

Summary

• Move merge policies (ExactMerge, IntersectMerge, and UnionMerge) to datumaro/components/merge directory

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch coverage: 80.92% and project coverage change: +0.01 🎉

Comparison is base (0277fa8) 78.89% compared to head (a291b58) 78.90%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #906      +/-   ##
===========================================
+ Coverage    78.89%   78.90%   +0.01%     
===========================================
  Files          208      211       +3     
  Lines        25037    25059      +22     
  Branches      5031     5031              
===========================================
+ Hits         19752    19774      +22     
  Misses        4152     4152              
  Partials      1133     1133              

Flag	Coverage Δ
macos-11_Python-3.8	77.89% <80.92%> (+0.01%)	⬆️
ubuntu-20.04_Python-3.8	?
windows-2019_Python-3.8	78.84% <80.92%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/components/merge/__init__.py	38.46% <33.33%> (ø)
datumaro/components/merge/exact_merge.py	52.50% <52.50%> (ø)
datumaro/components/merge/union_merge.py	82.97% <82.97%> (ø)
datumaro/components/merge/intersect_merge.py	93.15% <93.15%> (ø)
datumaro/cli/commands/merge.py	20.20% <100.00%> (ø)
datumaro/components/hl_ops.py	64.51% <100.00%> (ø)
datumaro/components/operations.py	86.11% <100.00%> (+3.47%)	⬆️

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