update simspec tutorial to use quickspectra

[sbailey]

Updates the simulating-desi-spectra tutorial to use quickspectra, while keeping the original tutorial as quickgen.ipynb. The new tutorial:

• generates 100 QSO spectra
• simulates those with with dark conditions using quickspectra
• runs redrock
• re-simulates with very bright conditions
• re-runs redrock
• compares redshift performance with and without moon (hint: moon is bad, though maybe not as bad as I would have expected).

External vetting welcome, or otherwise I will self-merge by Monday and we can have fun with lots of external vetting during the tutorial session.

Full disclosure: my git-fu failed me and I ended up re-adding the original tutorial under a new name instead of moving it in a way that would preserve history. For jupyter notebooks I don't think that is a big deal, but if someone feels strongly we could fix this properly in a different branch.

[moustakas]

This is very nice, @sbailey.

[dkirkby]

I agree this looks great! Some nitpicking suggestions in case you have time:

• "read out here" -> "read about here"
• (17.12?) -> (17.12 which will be available sometime in Dec 2017)
• %pylab inline -> %matplotlib inline (since you already imported np and plt)
• plot histograms of the sample redshifts and r-mag?
• add a line showing flux=0 in plotspec()
• will the srun command in cell [10] propagate the necessary DESI env?
• your moon-up geometry is non-Euclidean! moonsep=10 deg would fix this.
• if you want to see a bigger moon effect, increase the observing airmass.
• mention the mini tutorial in the appendix?

[sbailey]

Thanks for the suggestions; I implemented those in the notebook.

The srun command does propagate the notebook environment into the batch job, but it is possible that the user has commands in their .bashrc.ext, .bash_profile.ext, or .tcshrc.ext that could override that. I haven't mapped out the full phase space of what works and what doesn't, so for now I just added a note about what to look for if that command fails. I have tested this notebook with "clean" .bash*.ext files that do not configure any DESI code or environment variables by default (@tskisner finally convinced me that that really was the best thing...)