 Calendar model for a resource. It has

     - attendance_ids: list of resource.calendar.attendance that are a working
                       interval in a given weekday.
     - leave_ids: list of leaves linked to this calendar. A leave can be general
                  or linked to a specific resource, depending on its resource_id.

    All methods in this class use intervals. An interval is a tuple holding
    (begin_datetime, end_datetime). A list of intervals is therefore a list of
    tuples, holding several intervals of work or leaves. 