Clarify Charge Response of PMTs

[marc1uk]

This is something that could be better documented in the guide and code, but it's currently not very clear how PMT charge response is defined, or the units of output. The wiki has a guide on adding a new photosensor, but simply says the qpe0 array 'represents spe response' and copies another detector. The units and scaling, for input or output, are not at all clear, especially with the various magic numbers (https://github.com/WCSim/WCSim/blob/develop/src/WCSimWCPMT.cc#L75) involved in the process.

It would be good to be explicit about the entire calculation chain, the units involved at each step, any rounding, thresholds, and how the PMT specification should be determined from a given physical detector in order to be able to reproduce e.g. a test stand setup to ensure MC aligns with measurements.