add `constexpr` to ChannelIDs, and make constructors `explicit`

continuation of !2956 (merged): add as much constexpr as possible, and make single argument constructors explicit

as this sits on top of !2956 (merged), it (also) requires Rec!2361 (merged), Phys!886 (merged), Boole!332 (merged) In addition, needs Lbcom!546 (merged), Rec!2362 (merged) (which sits on top of Rec!2361 (merged)), Boole!333 (merged) (which sits on top of Boole!332 (merged)), Panoramix!86 (merged), Run2Support!10 (merged)