Completed task on subject area modelling of week-1

The merge request contains completed task "Training Center" on subject area modelling of 1st week. The task's link is https://g2.kzn.akvelon.com/net_internship/internship-2022/-/blob/master/week%201/OOP.md. Task: Design classes for modeling the subject area "Training Center". Your application should allow you to manage students, teachers, courses (subjects) and lessons.

Brief functional requirements:

1. one teacher can teach several courses
2. similarly, one student can attend several courses
3. there will be many lessons within one course
4. for each lesson, the teacher can give the student a certain score.
5. based on the results of the entire course, the student is also given the final score