Add DESI module

[dkirkby]

This issue is to add a bayez.desi module to encapsulate DESI-specific interfaces to bayez. I suggest the following functions:

• read_bricks(): Read b,r,z bricks and prepare for analysis by bayez.
• write_bricks(): Write b,r,z, bricks including truth using spectra generated by bayez simulation code.
• write_zbest(): Write bayez results to zbest file.
• desi_zfit(): Read bricks, perform redshift fits, and write zbest file.

[dkirkby]

The write_zbest() method should optionally write an extra BAYEZ binary table HDU with bayez-specific columns like z_median, z_mean, z_95_lo, z_95_hi, z_68_lo, z_68_hi, etc.

[fjaviersanchez]

Option added to write_zbest() called extrahdu. We still need to create write_bricks()