MuonTGC_CnvTools, MuonCnvToolInterfaces: Prepare TgcRdoToPrepDataTool* to be const.

Update TgcRdoToPrepDataTool and TgcRdoToPrepDataToolMT so that their interfaces can be made const.

Add IDC_Helper::addCollection as a thread-safe alternative to getCollection.

getCollection should be marked as not thread-safe, but this is commented out in this MR. Will be uncommented in a forthcoming MR .