Add comments and internal comments

[popescu-v]

• add comments to Dictionary, Variable and VariableBlock objects;
• add internal comments to Dictionary and VariableBlock objects.

Note: the position of the comments that are interspersed with dictionary information (e.g. before/after key, before/after metadata, etc.) cannot be reconstructed, because kdic file parsing gets the correct comment attachments to the entities they apply to, but not their textual position with respect to these entities. The attachments are sufficient for constituting the JSON dictionary representations though. Hence, semantically no information is lost upon DictionaryDomain object initialization from the JSON file that is generated by Khiops Core via the export_dictionary_as_json Core API function.

closes #423

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TODO Before Asking for a Review

• Rebase your branch to the latest version of dev (or main for release PRs)
• Make sure all CI workflows are green
• When adding a public feature/fix: Update the Unreleased section of CHANGELOG.md (no date)
• Self-Review: Review "Files Changed" tab and fix any problems you find
• API Docs (only if there are changes in docstrings, rst files or samples):
  • Check the docs build without warning: see the log of the API Docs workflow
  • Check that your changes render well in HTML: download the API Docs artifact and open index.html
  • If there are any problems it is faster to iterate by building locally the API Docs

[tramora]

Looks fine to me. I have just a question about the usage