Revisit how individual cameras are co-added in specsim results

[dkirkby]

Specsim returns per-camera vectors as well as co-added results. We currently only use the per-camera results in quickbrick and quickgen, but for SNR-level studies a co-add is still useful.

The coadd is currently throughput weighted:

obsflux = sum( thru[c] * resolution.dot(srcflux) ) / sum ( thru[c] )

where both sums are over cameras only.

Should we move to ivar weights, or even a full-blown spectro-perfect co-add (a la desi-doc-1056) ?

This is related to #1.

[dkirkby]

PR #35 removes the co-added output since it was not being used and needs to be re-thought once we have an actual use for it. I am leaving this open for now but adding the "enhancement" label.

[sbailey]

Although the specsim coadd isn't currently being used by the simulation pipeline, the heritage of specsim (i.e. quicksim) was originally all about providing a convenient single spectrum representing a reasonably realistic version of what DESI will produce, without having to involve the user in the messiness of individual channels. Similarly for e/BOSS, nearly all users work with the coadds (if they work with the spectra at all). In our drive to add more realism, we should be cautious about dropping that original goal.

i.e. ok to temporarily drop it, but let's not let the idea of perfect coadds get in the way of providing good enough coadds for non-expert users. Let's get coadds back in the next round of changes.