Group i.o. operations for reading dl2 event data in e.g. simtools/io_operations/dl2mc_reader

[tobiaskleiner]

Quoting comment by @GernotMaier that suggested to open this issue:
This is a very generic functionality which we will need more in futures: load a list of events from a DL2 file, etc. Should we consider moving this into a generic module? e.g. simtools/io_operations/dl2mc_reader or something like this?

I suggest for now to open an issue on this and don't change the code of this PR.

Originally posted by @GernotMaier in #1098 (comment)

[tobiaskleiner]

• Add NSB values to the dl2 fits files.

Current parameters in hdu (some of these could be used for further interpolation):

[
"runnumber",
"shower_prog_id",
"shower_prog_vers",
"detector_prog_id",
"detector_prog_vers",
"converter_prog_vers",
"shower_date",
"detector_date",
"primary_id",
"obsheight",
"num_showers",
"num_use",
"core_pos_mode",
"core_range",
"az_range",
"alt_range",
"diffuse",
"viewcone",
"E_range",
"spectral_index",
"B_total",
"B_inclination",
"B_declination",
"injection_height",
"fixed_int_depth",
"atmosphere",
"corsika_iact_options",
"corsika_low_E_model",
"corsika_high_E_model",
"corsika_bunchsize",
"corsika_wlen_min",
"corsika_wlen_max",
"corsika_low_E_detail",
"corsika_high_E_detail",
"corsika_low_high_E",
"detector_Simulator",
"combined_runHeader",
"fFADC_hilo_multipler"
]