TimeTable: refactoring of timetable for calendar

there was a small 'flaw' in the previous version, it could enter an
infinite loop ofr the calendar.

this version simplifies the whole process
TimeTable: delete useless field
TimeTable: remove useless line
TimeTable: correction after review
Merge pull request #601 from antoine-de/fix_cal_departure_board

TimeTable: refactoring of timetable for calendar