Refactor the photon list mechanism #84
Description
The PhotonList class in the mc module does not look quite right, as it is conflating the roles of the source and the detector.
We should have a photon list class in the source module, and Source.rvs() should return a PhotonList object.
A Sensor object should then be able to operate on a PhotonList and return a more elaborated class (EventList), with the addition of the conversion z coordinates and the number of electron-hole pairs.