`add` method replaced with `insert`.
`insert` and `select` implemented for new database base class.
Database unit test added.
Due to SQLite implementation of the primary key autoinc, the type of the
IDs on the database for SQLite changed to Integer.
https://www.sqlite.org/autoinc.html
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.
