-------------------
Implements real-entities (21)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This is a major refactor of the code, including changes to database structure
and the way data is imported. The main changes are:
1. Entities now register the source of the data.
This was necessary to detect easily cross source references,
such as xsame_as that link occurrences in different
sources refering to the same entity.
2. Migrations with Alembic are now used to update the database
schema. This is a major change, and it is still experimental.
From this version onwards changes to the database are handled
through Alembic and authomatically
triggered when a Timelink database is opened
with TimelinkDatabase() class.
3. The import process was refactored to save the context of
cross source references, and restoring them after reimport.
`See this note <https://time-link.github.io/timelink-docs/D_Updating_sources_in_the_database/D.1%20Processing_new_versions_of_source_transcriptions/>`_
4. Improvements to to_kleio() rendering, taking into account
`extra_info` with comments and original wording.