Refactor: edit commenting, TODOs

Some of Charlie's comments removed (no offence, but commenting getters
& setters is simply clutter), others edited, others added.

Added an omitted `this.` missing from `results` in Student(), too.

Student does not enforce anything on the values it stores, except that
they are integers: this is correct behaviour. TestDatabase should use
Students in a way it likes, rather than Students being made for
TestDatabase. Commit coming shortly.