Add UserWarning if an invalid media_type comes to image statistics computation

[vinnamkim]

Summary

• Error using datum stats #873, we found compute_image_statistics() that just raises this error:

AttributeError: 'PointCloud' object has no attribute 'size'

• However, this gives no hint for an user about what is going on behind.
• We should inform the user that image statistics cannot be obtained because the user's dataset contains media of a type other than image media.
• Refactor compute_image_statitiscs test to use pytest framework too
• Ticket no. 106634

How to test

This change is covered by the unit tests.

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: 100.00% and project coverage change: +0.01 🎉

Comparison is base (7278c8c) 78.61% compared to head (89e49c6) 78.62%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #891      +/-   ##
===========================================
+ Coverage    78.61%   78.62%   +0.01%     
===========================================
  Files          205      205              
  Lines        25067    25076       +9     
  Branches      5029     5031       +2     
===========================================
+ Hits         19706    19717      +11     
+ Misses        4229     4228       -1     
+ Partials      1132     1131       -1     

Flag	Coverage Δ
macos-11_Python-3.8	77.61% <100.00%> (+0.01%)	⬆️
ubuntu-20.04_Python-3.8	78.62% <100.00%> (+0.01%)	⬆️
windows-2019_Python-3.8	78.56% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
datumaro/components/operations.py	83.97% <100.00%> (+0.29%)	⬆️

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