xAODMuonPrepData - Split up the sTgcPrepData

Hi everybody,

this merge request splits up the sTgc uncalibrated measurement container into Strip, Wire and Pad hits because I think they're quite differnt

• Strip hits are the only ones which are clustered. They're the only one where the information about the cluster constituents makes sense
• Pad hits are intrinsically 2D measurements. Dumping their local coordinates is incompatible with the dump of strips and wires

Tagging: @emoyse, @pscholer