Multi-threaded version of the simulation code

Warning: this is a slightly simplified version of the code compared to MAIN.py. I have commented a few things and I also made sure to save appropriate information for the subsequent photometry code that were previously missing. I let @carlotal decide if this version should become the reference or just a handy tool to generate quickly big data sets. Note however that maintaining MAIN.py and MAIN_multiprocess.py in parallel might be difficult. I struggled a little bit when @carlotal was doing updates to MAIN.py.