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>
The models that are defined in the pull request should be seen as internal components that we will further reuse as our own abstraction layer on top of the actual records. As we discussed some time ago, the graph that will be built internally by the disambiguation algorithm will eventually be saved into the records themselves, using the relationship mechanisms that will be decided. Since we are not there yet however, we would like to keep this PR like this for the moment, in order for us to proceed with the next features (starting with #2569). When #2573 will be agreed, implemented, reviewed and merged, we will be glad to simplify our own models if we can. What do you think @kaplun @jirikuncar ?