Merge commit '843a644b617a2262a7677ef98d3a06f55b3519a4'

Conflicts:
	libsmo/include/learn_period.hpp
	libsmo/include/lesson.hpp
	libsmo/include/smo_user.hpp
	libsmo/src/grade.cpp
	libsmo/src/learn_period.cpp
	libsmo/src/lesson.cpp
	libsmo/src/smo_user.cpp
Altered code to conform changes in the API
(removed copy subjects from other learn period feature)