Move the token creation/renewal code into the NordigenClient and remove
external access. The NordigenClient now also takes the DB client in the
constructor.
While creating the unit test, noticed that the nordigen mocks for
downloaded transactions could match the simple transactions mock, thus
helping in the online download command.
Also created the nordigen.NordigenClient mock, with the used methods
mocked and raising a NotImplement when a new endpoint is requested.
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.
