Pass RPC y/z dimensions correctly to ReadoutElement

Hi,

this MR fixes the LocalGasGapPos computed here doesn't match the one retrieved from the GeoPhysVol for rpc RE WARNING when processing Run3/4 layouts with BI RPCs by explicitly passing the y- and z-translations of the 3-gap RPCs to the RpcReadoutElement. The remaining issue to investigate is the offset in the x-coordinate of 0.74 between the GeoModel description and the manual calculation. Will open a ticket on that one after this MR is merged.

Best, Nico