Add deprecation warning for ITPParser

[lilyminium]

trafficstars

Fixes #4698

Changes made in this Pull Request:

• Adds deprecation warning for element guessing in ITPParser

Please feel free to make any changes desired and push into this branch!

PR Checklist

• [x] Tests?
• [x] Docs?
• [x] CHANGELOG updated?
• [x] Issue raised/referenced?

Developers certificate of origin

• [x] I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4744.org.readthedocs.build/en/4744/