Adjust JetUncertainties mass-scale accessors to be created where they are used

@janders encountered a problem when trying to implement the recent jet recommendations which was traced back to how the accessors were being created. I'm not sure why it worked for some users but it didn't work in SUSYTools, but either way it's good to fix this problem. Anyone who already has the recent jet recommendations working is unaffected, while those who were getting auxdata exceptions when using the tool should no longer receive such errors.