added some functions for ClassDAO and EnrollmentDAO
Merge branch 'master' of https://github.com/eng7miky/GradSys

Conflicts:
	GradingSystem/GradingSystem/DAO/CourseDAO.cs
	GradingSystem/GradingSystem/GradingSys_DataClasses.dbml.layout
	GradingSystem/GradingSystem/GradingSys_DataClasses.designer.cs