Set threshold
Fix a bug where the background uncertainty was not being calculated properly

The background uncertainty is defined as follows:
    * Determine the corrected yield using the nominal background,
      the upward fluctuating background and the downward fluctuating
      background
    * Take the largest difference between the fluctuations and the
      nominal value
    * Determine the relative uncertainty as largest difference / nominal
      corrected yield.

This recipe was not followed, leading to artificially large uncertainties