SUSYTools: Handle fatjets systs properly in TStore

SUSYTools' SUSYObjDef_xAOD::GetFatJets() function doesn't properly handle recording fatjet containers of systematic variations in the xAOD::TStore. They would all get the same key in the TStore, resulting in the TStore complaining about Trying to overwrite object with key <the_key>

Trivially fixed by putting the name of the systematic variation into the key, similar to how it is being done in all other SUSYTools Get functions, e.g. SUSYObjDef_xAOD::GetJets() or SUSYObjDef_xAOD::GetTrackJets().

Tagging @salderwe and @janders so they can approve this small change :-)