Lab.it1 = Refactor: +CourseBase +Lab

Course now inherits from the CourseBase class, behaves exactly the same.
(all orig course tests still pass). Created Lab class. Added labs list
to Course for further development.
Lab.it1 - fail: add lab
Lab.it1 - pass; Lab.it2 - fail: return last
Lab.it2 - pass: return last
Lab.it3 - pass: next; Lab.it4 - fail: prev
Lab.it4 - pass: prev
lab.it5 - pass: overflow