Several fixes

A container branch for the fixes of several issues I encountered while rolling out the current SaQC on the SEEFO pipeline.

Not sure, if this needs a real review, but as some of the changes touched code I am not completely familiar with, maybe you, @palmb and @luenensc want to have a look anyhow?

saqc/core/flags.py

 #!/usr/bin/env python
-
 from __future__ import annotations
 
 import pandas as pd