- change TEXT to VARCHAR (or issue with postgres)
- clean some classes where DeclarativeBaseGUID was used instead of DeclarativeBase
- add a create_book function to create new books (require SQLalchemy_utils) by generating the schema and then removing the foreign keys
- return sessions (wrapped with some more attributes) instead of books (as these could detach themselves of the session)
- refactored model_core into a submodule
- clean the open_book function
- adapt the sandbox examples
- fix doc to include modules