Ensure that the MagFieldSvc is properly set up when running TRT-only Digitization
Make the ServiceHandle a member of TRTDigitizationTool
rather than
TRTProcessingOfStraw
(not a component). Otherwise we rely on some
other component having a handle to the MagFieldSvc and initializing
it properly.
(Should not change the output of standard digitization jobs where other
sub-systems retrieve the MagFieldSvc.)