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.
since yaml.load() is deprecated, and the loaded stuff does not need
to be complex anyways, use yaml.safe_load(). This gets rid of the
deprecated yaml.load() method and uses the safe approach for loading
yaml files
You can now place a file name '.courseignore' in the ROOT directory
and specify - in each line - the name of a directory. Directories named
this way and located in the ROOT directory will not be treated as course
directories
