Put all exception in package exceptions and rename BaseException to MonliftException
Refactor code affected by the modification of exception locations
Refactor entities so we stick to the datastore Parent-Children representation of data
Update factories and manager to retrieve entities using the parent relationship
Update the way validation of requests is done. Implemente

Implemented the visitor pattern for the validation request. The
RequestValidatorVisitor is now solely responsible for validating request
Added LiftMarshaller and update marshaller to return list of response objects
Updated the function promoteUserToDriver and everything connexe

Also added a function to get user by their username