General UI improvements

[duembgen]

Collection of notes by @holmesco and myself to improve the usability

• get_Q should only return Q, not Q and y.
• PolyMatrix.init_from_sparse() should return only matrix, not matrix and var_dict.
• make only get_x an abstractmethod, because get_p shouldn't have to be defined when no parameters are required.
• there is an assumption that there will always be landmarks. This should be removed.