Session_03: Issue 2.b) Refactor CSVDatabase to be a singleton

[LukasStryg]

As a developer, I want to refactor the CSVDatabase class to be a singleton, to ensure that there is at any given point in time only one CSV database.

Acceptance criteria:

• Refactor CSVDatabase to adhere to the singleton design pattern.

Definition of done:

• All acceptance criteria has been fulfilled.
• Only one CSVDatabase in the application.