A course now does not handle its lectures directly, but has a
'script' property. The script essentially works as the course before,
except that some methods have been moved from the lectures class to the
script class as well.
This ensures further modulation of the course class, to introduce e.g.
an exercise class in the future that a course can also have.
Additionally, with this come further configurations in the info.yaml
file so that the script can have a different relative path from the
course, and the lectures a relative path from the script.
