Add the TopicInfo data structure

[Stannislav]

Fixes #539 .

Description

This adds the TopicInfo data class that replaces the manual creation and copy-pasting of the structured dictionary that we have been using for this purpose so far.

How to test?

• See new tests under tests/unit/database/test_topic_info.py
• Adjusted tests for old code pass

Checklist

• This PR refers to an issue from the issue tracker.
  (if it is not the case, please create an issue first).
• Unit tests added.
  (if needed)
• Documentation and whatsnew.rst updated.
  (if needed)
• setup.py and requirements.txt updated with new dependencies.
  (if needed)
• Type annotations added.
  (if a function is added or modified)
• All CI tests pass.

[EmilieDel]

Really nice 🎉 It is definitely more clear and clean than copy-pasting the dictionary for every source, thanks for the changes!

[jankrepl]

Very reasonable! Thank you for the effort:)

[FrancescoCasalegno]

Discussed 2022-01-11

Let's wait for #535 and #540 to be merged into master before merging this PR.
Notice that the code will need to be changed to address the updates on the master.

[FrancescoCasalegno]

Thank you for your work @Stannislav Stan!

It will definitely make our code simpler to maintain and understand!

[FrancescoCasalegno]

Lgtm!