allow only one input option in MGUIOptionsEnergyCalibrationUniversal

[Emily-Broadbent]

Currently, in the MGUIOptionsEnergyCalibrationUniversal file, it allows both the ThresholdValue checkbox and ThresholdFile check box to be applied at the same time. Should be changed to allow one or the other, not both

[ckierans]

Relatedly, the current GUI doesn't allow for a threshold value other than 35 to be entered. I'm assuming Andreas's changes in PR #75 will address this, but it needs to be checked.