TrigOutputHandling: use OrderedSet semantics in TriggerEDMSerialiserTool

Frank Winklmeierrequested to merge
fwinkl/athena:sertool into master

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).

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