More consistent use of combineReduce, simpler and/or reductions

• update code base to reflect the consolidated list/map reductions
• replace old code use of returnReduce(list.size(), sumOp<label>()) in favour of returnReduceOr(list.size()) which is cleaner to write/understand and also marginally more efficient

General overview under https://develop.openfoam.com/Development/openfoam/-/wikis/coding/patterns/parallel

• broadcast
• reductions
• more reductions