Tutorial json
    name            TutorialName
    course          CourseId
    type            (CI Text) -- "Tutorium", "Zentralübung", ...
    capacity        Int Maybe -- limit for enrolment in this tutorial
    room            Text
    time            Occurences
    regGroup        (CI Text) Maybe -- each participant may register for one tutorial per regGroup
    registerFrom    UTCTime Maybe
    registerTo      UTCTime Maybe
    deregisterUntil UTCTime Maybe
    UniqueTutorial course name
Tutor
    tutorial    TutorialId
    user        UserId
    UniqueTutor tutorial user
TutorialParticipant
    tutorial    TutorialId
    user        UserId
    UniqueTutorialParticipant tutorial user