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.)

CHANGELOG.md

@@ -17,6 +17,7 @@ This changelog starts with version 2.0.0. Basically all parts of the system, inc
 ### Changed
 - documentation pipeline changed to base on methods decorators
 - `flagOffsets` parameters `thresh` and `thresh_relative` now both are optional
+- flags concatenation tasks (for squeezed and explicit histories) are now all channeled through the function `concatFlags`
 - corrected false notion of *residual* concept (old notion: *residue* got replaced by *residual*)
 ### Removed
 ### Fixed