StiIstDetectorBuilder: Get sensitive IST volumes from the global TGeoManager in a manner similar to PXL and SST detector builders.
StiXxxDetectorBuilder: Minor code juggling
StiIstDetectorBuilder: Removed pointless check for valid gas surrounding detectors in this builder. The gas is set right in this class anyway
StiIstDetectorBuilder: Renamed a bunch of local variables just to be consistent with the other detector builders

Also updated the output sent to LOG_DEBUG for the same reasons
StiPxlDetectorBuilder: Removed unutilized code