CPAlgorithms: expose the useClusterEta property of the PtEtaSelection tool

The CP::AsgPtEtaSelectionTool set up by the PtEtaSelectionBlock has an option to use the cluster eta instead of the track eta: this is the recommended approach when applying eta cuts on electrons and photons. This MR exposes that option at the level of the config block.

I'm not sure whether we want to enforce the use of cluster eta on egamma objects by default (or how best to do it). Currently, we expect the user to know that this option is needed for electrons and photons, but not for jets, muons or taus - is that a reasonable expectation?