Make SaQC immutable again

Everything should now be immutable again. Unfortunately we still have a memory leak, i.e. every function call increases the memory footprint of the SaQC object. Interestingly, this increase is produced by saqc._data and not saqc._flags, as I would have expected. Initializing new DictOfSeries from existing ones seems to leave some unfreed memorey behind. I tried to dig into this, but couldn't find any (semi-)obvious reasons, that's why I'd stop the search for now.

Anyways the increase in memory usage seems to be acceptable: With data of size (10, 5_000_000) and a single test on every column increases the memory usage from ~1_730_000 to ~ 2_094_000 in steps of ~20_000 bytes per function call.