simplify/unify flags concatenation workflows

Implemented changes as suggested in #305 (closed)

• I have some problems figuring out the right way of squeezing flags as suggested in #305 (closed). I left a TODO note in the respective code line (funcs.resampling.concatFlags) and are requesting help from either one of you @palmb / @schaefed . (Or finlly a comprehensive documentation of the many pitfalls of history/flags object handling. I am really a bit lost any time i have to deal with those objects.)

saqc/funcs/resampling.py

@@ -490,7 +490,7 @@ def concatFlags(
              'match'}, default 'match'
         The method used for projection of ``field`` flags onto ``target`` flags. See description above for more details.
 
-    freq : {None, str}, default None
+    freq : str or None, default None
         The ``freq`` determines the projection range for the projection method. See above description for more details.
         Defaultly (None), the sampling frequency of ``field`` is used.
 
@@ -551,7+551,7 @@
 
     elif method == "match":
         func = lambda x: x
         func_kws = {}
 
     else:
         raise ValueError(f"unknown method {method}")