Support multiple evaluation tasks

[ahumenberger]

As of now, we only have a single evaluation task, that is, "provide tests for this code". In a future release we want to run other tasks as well, like "repair this compilation error". For this, we need structural changes.

Tasks:

• Introduce string enum TaskIdentifier
• Change Model interface such that it has a RunTask(TaskIdentifier,...) method
  • Remove GenerateTestsForFile from Model interface as this becomes a task
• Add a method IsTaskSupported(TaskIdentifier) to the Model interface to let models specify if they support a specific task.
• Introduce proper Repository type
  • Each repository can allow/disallow certain tasks
    • This should not be hardcoded. Therefore a repository.json file can be created in the repository's root directory. If no such file exists, all tasks are allowed.
• Create Task interface and suitable implementations
  • TaskPostRunTests which calls Model.RunTask and then runs the repository tests to validate the result