Refactor duplicate code

[vandalt]

This reorganizes the code so that:

• Functions used to calculate specific quantities are all grouped in a separate file
• The main code to run the ETC (which was in the two scripts) is in a separate "driver" file, inside a function, and called by the scripts. This fixes Reorganize duplicate code in NIRPS_ETC.pyand NIRPS_ETC_script.py #2.
• Some formatting updates (spaces around comments, equal signs, commas, line spacing, and other minor formatting changes)
• Plots can now be saved even when running the script in a loop, but the option is off by default.

This is mainly to make the code easier to edit and debug. The changes seemed to improve a bit execution time (but just by ~ 0.1 s). The results with the default settings seem to be the same as in the main branch.

@ngrieves let me know if you have comments or suggestions.