Add optional columns for MC generated events

[TarekHC]

The parameters of MC generated events should be added as optional columns, to store the true energy, direction and particle ID of each event (required to calculate IRFs from event lists).

[maxnoe]

Also the required columns would need to be different, as most simulations won't have well-defined RA / DEC / TIME

[TarekHC]

I would say the most basic parameters would be MC_DETX, MC_DETY and MC_ENERGY, given that the main rationale to generate DL3 MC files will probably be effects over the FoV.

[kosack]

I suggest using the terminology true_x rather than mc_x which is jargon.