Heavy memory requirements of truth simulations

[ramirezdiego]

#99 was successfully tested and merged, but still provides an expensive solution for building the truth output. The propagated photons, which carry information for the contributing cluster of our events, need to be stored on disk to later on run the truth plugins. These targets (propagated_s2_photons and propagated_s1_photons), not stored by default, have huge sizes and can write TBs of data within minutes, as also reported by @cfuselli. We might want to optimize this.