Truth xAOD generator filters
This MR brings couple of new generator filters that run over the xAOD::TruthParticles instead of the HEPMC McEventCollection. The new filters are just a translation of the already existing filters (ex. MissingEtFilter -> xAODMissingEtFilter) and the goal is that once the xAOD filters will be fully validated they will become the default ones. The novelty with the xAOD filters is that they run on slimmed TruthParticles containers (TruthMuons,TruthElectrons, etc) in order to have the filter implementation as clean as possible. For each type of particle there is a corresponding slimmer that will decorate the particles with the needed properties. Before the usage of the xAOD filters it's require to call the xAOD convertor and the creation of the necessary containers with the slimmers. This is done by including GeneratorFilters/share/common/CreatexAODSlimContainers.py in the JobOptions before the filter part.