ATR-29081, AFT-726: more work on conditional b-tagging decoration

This is a followup on !73321 (merged). Changes:

• Use a std::set property to translate from a python set
• Use std::vector::assign [std::map::insert] to transfer the double default decorator values to float. Right now there's a loop that several people commented on (all agreeing that this wasn't a major point), but we could just avoid the loop completely.
• Come up with better default values for GN2v01. I just said "light jet" but I vaguely recall that we should probably call it a tau. Best would be to get exactly the values that GN2 uses and therefore not even change outputs at all.
• Rename the flag to enable this (probably need some discussion on what we'd call it) and possibly change the type.
• rename conditions to something else.
• move the join function into a common place.