TrigOutputHandling: use OrderedSet semantics in TriggerEDMSerialiserTool
Use OrderedSet
semantics for the "CollectionsToSerialize" property
instead of defining our own merge semantics in Python. Note that we
could have also just used a set::set
in C++, which would have the same
effect (but the Python type would still be list
, which is a bit
confusing maybe).