TrackVertexAssociationTool Update, master branch (2021.05.11.)

As described in ATLASRECTS-6317, in a RAWtoALL reconstruction job the usage of SG::ReadDecorHandle variables in CP::TrackVertexAssociationTool was showing up as a bottleneck in threading tests. See:

As suggested by @ssnyder on JIRA, I introduced SG::AuxElement::ConstAccessor variables as members on the tool, and updated the code to use those instead of temporary SG::ReadDecorHandle variables. This made the tool completely disappear from VTune's "top offenders list".

Beside replacing all ReadDecorHandle objects with ConstAccessor ones in the code, I also removed all __attribute__((unused)) declarations from it. Replacing them with a much more platform independent way of not triggering warnings about unused variables in the first place. (I just don't like using such language extensions when there isn't much need for them.)