Refactor FlightTutorial, FuelScoopTutorial, StationLandingTutorial, TemplateTutorial as classes

[BarthPaleologue]

Right now the tutorials are implemented as global objects, which I want to avoid where possible.

Here, it is quite straightforward to refactor them as classes implementing the Tutorial interface and instantiating them where they are used. Having classes may also allow to create variations around these tutorials in the future.

The save loading and parsing can then be moved to their tutorial's respective constructor.

Also adding the license header in those files would be a nice improvement!