Integrate abstract into sentences table

[EmilieDel]

Fixes #579.

Discussion

• Abstract is divided into several parts (parser is extracting a list of paragraphs). In the current PR, everything is saved under one paragraph with section Abstract. Should we instead keep the different paragraphs separated ?

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.