Scale down Castor photoelectron gain at digi step to avoid saturation in simulation of PbPb

Castor calorimeter pmt gain, photoelectronsToAnalog, set to 4.24 divided by 20 to work around saturation of digital signals in PbPb simulation; this assumes also a corresponding check of conditions (previous setup was 4.24 / 5 and was valid for pPb)