Refactor mixed catalog client requests

[faysou]

Pull Request

Refine mixed catalog client requests

• Allow to extend a catalog after its last datetime or before its first date time
• Automatic determination of append and prepend catalog write mode
• Note: to avoid creating holes in the data, priviledge only using start date and not end date in data requests when updating the catalog as well. Update: I've added a condition enforcing not using both end and update_catalog_mode at the same time.

Type of change

• New feature (non-breaking change which adds functionality)

How has this change been tested?

Existing tests passing, more tests needed, possibly after a merge. Found and fixed bugs to make existing tests work. Also the key logic is concentrated in _handle_date_range_request and is easy to understand.