DESI degraders

[BStoelzner]

I started implementing a simple degrader that applies a DESI ELG photometry cuts in: https://github.com/LSSTDESC/rail_attic/blob/BStoelzner/degrader_in_progress/src/rail/creation/degradation/spectroscopic_selections.py
At the moment it applies cuts on g,r, and z as given in Raichoor et al. and a selection based on the success rates as a function of colour given in Fig. 18.
In order to mock up a realistic sample we've identified the following tasks:

• Our input catalogues (cosmoDC2/Skysim 5000) do not provide all required magnitudes: g_fib (for ELGs), WISE colours (for LRGs, QSOs), etc. These need to be derived from the input catalogues
• Implement a selection of galaxies based on the colour-redshift distribution in the SV data
• As previously pointed out by @janewman-pitt-edu this selection method might not be sufficient for the quasar sample, which will require some forward-modelling.
• Discreteness effects might be an issue, at least when using cosmoDC2
• We want to separate the selection of galaxies with successfully obtained redshift and the selection based on colour because they take different inputs

[aimalz]

This is waiting on LSSTDESC/rail_base#33.