Add some input
Use string interpolation
Make @courses available from outside
remove "course" from initialisation

a student in no longer needs to have a course from the beginning on. I instead added Join and Leave methods, and the courses are saved in an array in @courses
Add methods to join and leave courses
Add enlist and drop_out methods to courses

i also made @studentinnen available from the outside, so that the array it returns (i.e. the studentinnen enrolled in a course) can be called from the outside (e.g. puts kurs2.studentinnen.count)

#enlist corresponds to #join_course in Studentin, they get called simultaneously, that whenever a course gets joined by a student, the student also joins the course, which means that both Course.studentinnen and Studentin.courses get a new element.
#drop_out corresponds to # leave_course in Studentin. see above