Coadd arrays not sorted by fiber number

I don't see any datamodel doc for the SV0 coadd files but I notice that fibers in the BRZ_* arrays are not sorted in order of increasing FIBER number, so permuted relative to arrays in (C)FRAME files. Is this deliberate or could we preserve the (C)FRAME ordering to make it easier to match the individual spectra contributing to a coadd?

To reproduce this feature / problem:
```
# FIBER order in CFRAME spec 0
fitsio.read('/global/cfs/cdirs/desi/spectro/redux/daily/tiles/66003/20200315/cframe-b0-00055655.fits',
ext='FIBERMAP', columns='FIBER', rows=range(10))
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], dtype=int32)

# FIBER order in COADD spec 0
fitsio.read('/global/cfs/cdirs/desi/spectro/redux/daily/tiles/66003/20200315/coadd-0-66003-20200315.fits',
ext='FIBERMAP', columns='FIBER', rows=range(10))
array([365, 471, 459, 356, 474, 398, 191, 397, 469, 363], dtype=int32)
```

Start by comparing the referenced cframe and coadd FITS files, especially the FIBERMAP FIBER column and BRZ_* arrays, using the shown fitsio reads. Determine whether the differing order is deliberate; done means either preserving the (C)FRAME ordering or documenting the intended coadd ordering and its matching implications.

Written by the indexing model from the issue text.