Make sure SaveLLHHisto is initialised for MMC
Follow up to !75101 (merged)
As the m_SaveLlhHisto was never initialised, this was introducing some random irreproducibilities. Now fixed by setting it to false by default in the MissingMassCalculator constructor.
Also moves the declaration of the histograms for both m_fUseFloatStopping + m_SaveLlhHisto to the setter functions, otherwise they would only be configured based on the default values in the constructor.