Adds a new table, nordigen, with the access and refresh token, along
with their validity.
The Export/Import test would raise an integrety with the use of a real
DB and the export of the transaction IDs, so add a try-except block to
the database session to catch the error and re-raise an ImportError.
The new command ImportCommand takes a Serializable type, from which it
can call the deserialize method to generate a DB ORM type. The
Serializable interface also declares the serialize method.
(De)serialization moved to the ORM types, due to the inability to
properly use overloading.
Possible improvement for the future is to merge serialization
information on JSONDecoder/Encoder classes.
Adds a MockClient with the in-memory SQLite DB which can be used by
tests.
Most types export/import functionally tested using two DBs and comparing
entries.
