WIP authors: initial release

* Initial module structure.
* Introduces SQLAlchemy models for Publication, Author and Signature.
* Introduces claiming operations on signatures.
* Adds citation association table.
* Tests for models.

Signed-off-by: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Co-authored-by: Gilles Louppe <gilles.louppe@cern.ch>
Co-authored-by: Mateusz Susik <mateusz.susik@cern.ch>
Co-authored-by: Evangelos Tzemis <evangelos.tzemis@cern.ch>
this might cause this issue:
```
sqlalchemy.exc.InvalidRequestError: One or more mappers failed to initialize 
- can't proceed with initialization of other mappers.  
Original exception was: Property 'publication' is not an instance of ColumnProperty 
(i.e. does not correspond directly to a Column).
```