Extend simulation.py. Implement features necessary for the simulation of high energy cascades.

[fschlueter]

This PR adds several comments which add features to simulation.py

• 78c965054988a29bc70d209bbc99ee0be5ea388a allows to simulate daughter particles of (high energy) electrons necessary to simulate the longitudinal extend of cascades
• b5a6adbbe0a5de461c6173f788b18005ab70366f implements a pulse merging which significantly (~ x10) reduces the event size. A similar flag exists for the EHE simulation chain with photonics. Here, I have open questions regarding whether one should mimic the exact sampling resolution of the detector or not.
• a68da597ddd87e24046223ff2c8e245e7aedea26 allows to store the mean charge prediction for every DOM in a frame.

The remaining commits do minor improvements in logging and commenting.

[fschlueter]

@mhuen what about this PR?

[fschlueter]

@mhuen ping