Self-contained schedule model

[datarttu]

The same way as the network model, the sched schema should be made "independent", such that it could be given data from many different sources, and the schema itself would enforce the data integrity and business rules.
Some modification history model would be useful here as well.

In the near future, schedule data will probably be imported from sources other than GTFS, since the GTFS methods have turned out to have useless "detours" to a large extent.
We should be able to use Jore and / or DW instead.