Question engine: report methods should not require a list of slots

In the question engine, question_engine_data_mapper class in question/engine/datalib.php, there are methods like load_questions_usages_latest_steps, load_questions_usages_question_state_summary which require you to pass in a list of slots that you want data on.

This makes sense in the context of mod_quiz, when there is always a fixed quiz structure, but in other scenarios (we are currently working on https://github.com/moodleou/moodle-report_embedquestion, and this probably also affects mod_studentquiz) there is no fixed list.

In the other method load_average_marks, the $slots parameter is optional, in which case it returns data on all slots.

I think we should change these two methods, so that $slots is always optional. This will be a backwards-compatible change.