Moved the virtual grade() methods from the header file to the source file.
 On branch master
 Changes to be committed:
	modified:   bin/Debug/schoolMarkLogger
	modified:   bin/Release/schoolMarkLogger
	modified:   competence.cpp
	modified:   competence.hpp
	modified:   lesson.cpp
	modified:   lesson.hpp
	modified:   quartal.cpp
	modified:   quartal.hpp
	modified:   quit_exception.hpp
	modified:   school_year.cpp
	modified:   school_year.hpp