IDEA/FEATURE: allow (gtfs2db?) to add and/or delete based on date- (calendar or calendar_date)

[vingerha]

Use case: datasources update with a certain frequency, from what I have seen this is anywhere between weekly and annually. Esp. for the high frequent updates there are cases that the new source is already published before the current data expires and there is no overlap. When performing a nightly automated update one can have a situation where the new datasource can not yet be used as its first data is e.g. tomorrow. Options (not exhaustive)

• donot unpack if the new data contains only future dates
• add new data to current database based on date, i.e. combine sources
• delete from db based on date, this would also assist in removing past calendar (dates) and trips, etc. with the idea to keep the db small and performing