DataHandleHolderBase: define semantics for ExtraInputs/Outputs

Frank Winklmeierrequested to merge
fwinkl/Gaudi:dhbase into master

Define OrderedSet merge semantics for the ExtraInputs and ExtraOutputs properties. The C++ property type is std::unordered_set, so we don't strictly need it to be "ordered" but it does not hurt either (and we do not have a regular set merge semantics defined).

cc @tbold

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