Reorganize duplicate code in NIRPS_ETC.pyand NIRPS_ETC_script.py

[vandalt]

There seems to be a lot of duplicate code between the two scripts. This does not affect the results, but to make the code easier to maintain, I can try to put the code common to both in a separate file in the next few weeks.

[ngrieves]

Yes, these codes are basically identical except the script loops through the targets from the input file, this was just a fast way to create one code that outputs the plot and SNR/order and another that just gives overall SNR and RV values for a number of targets. Due to the time crunch I didn't remove any duplications. That sounds good though thank you Thomas