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.
Due to the use of the dataclasses mixin on the SQLAlchemy types, a
back_populates creates a RecursiveError when comparing two types. This
occurs because the dataclass will overwrite the __eq__ operator, and it
doesn't know when to stop comparing relationships.
Removing the dataclasses isn't the best approach, since then __init__,
__eq__ and __repr__ methods would have to be added to all types. Thus
the solution was to remove the relationship on the child (on a
one-to-one relationship) from the __eq__ operation, with the use of the
compare parameter.
Took the opportunity to define more logical __init__ methods on the
`Rule` and child classes.
Also revised the parameter options on some DB types.
Having a comprehension unit testing allows for peace of mind
refactoring.
Changed the PSD2Client constructor and extract method to accept
credentials and banks, respectively.
Create Exception tree for the Extract module.
Adds mocks to the NordigenClient requests' methods.
