FIX-FIX-FIX: delayed SSP

I better not explain what was wrong here ;-)
Let me rather summarize what we now have:
- clean API with proj='delayed' arg + RuntimeError if necessary preconditions aren't met
- improved support for non-SSP workflows
- more confidence that all data access streams result in the desired results
-- data are always returned preprocessed, whether delayed or not, but the preprocessing is done in different places.
-- iteration and get_data get now do the same
-- calling get data before and after dropped bads returns the same data

...