new desi_group_spectra

[sbailey]

Opening this PR for review while I do the final cleanup (e.g. adding unit tests).

This PR is a complete rewrite of desi_group_spectra for several reasons:

• propagates SCORES tables from cframe files into spectra files
• 40x faster by using fitsio, optimized data appending, and smarter "do I really need to do this frame?" logic
• isolates "which spectra files need to be updated" logic into a single function, which could be replaced in a future update with a spectro-processing DB query

Known issues I want to fix before merging:

• no unit tests (I believe that was true of the previous version too)
• --nights option became --night (easy to fix)

Known issues that may wait for a further update.

• appending a new night that is completely missing one spectrograph channel for
  every frame, when the prior data did have that spectrograph channel.

In the name of optimization, this bypasses the standard Frame and Spectra objects and deispec.io.read/write routines. The FrameLite and SpectraLite objects here might be useful for other purposes (like QA) but are currently isolated into pixgroup.py and are not intended to be generally useful objects with the full features of Frame and Spectra. They are optimized for fast I/O and regrouping, but need unit tests to help ensure that they remain consistent with the standard more full-featured objects.

[julienguy]

• Pushed a modif to create output directory if missing
• Tested on month simulation (5 nights, 827 spectra files). Ran successfully in 5min using 5 nodes on cori.

[sbailey]

I changed the option back to --nights and added unit tests (and satisfyingly fixed a few bugs that were found...). Thanks for the check @julienguy. I'm ready to merge when travis tests pass.