Changed:
When we want to add a group, if there exists CourseGroup with the same (CourseOccurence, class_type, mark), this group will be updated and wont be added along side.
For the classes: those who didn't changed will not be changed, those who don't exist in the db will be added, and those who exist and not to be added - they are irrelevant and will be deleted.
For issue https://github.com/asaf-kali/coursist/issues/8
Changed: When we want to add a group, if there exists CourseGroup with the same (CourseOccurence, class_type, mark), this group will be updated and wont be added along side. For the classes: those who didn't changed will not be changed, those who don't exist in the db will be added, and those who exist and not to be added - they are irrelevant and will be deleted.