Fix desi_quicklya.py

[andreufont]

This short code in desimodel/bin/desi_quicklya.py is used to generate Lya SNR files that are then used in Fisher forecast.

I added this code two years ago, heavily based on quicksim.py by David Kirkby. However, it doesn't work now since it is trying to load an old module (desimodel.simulate) and the original, and I believe the original code quicksim.py does not exist.

It is not a priority, but it would be nice to fix at some point.