Spectra coaddition

[julienguy]

Add functions and a script to coadd spectra. The intention is to add this as a pipeline task.
This inherits from the work of N. Busca, D. Kirkby and others, with large improvements on CPU time when resampling to another wavelength grid using 1D spectro-perfectionism and the divide and conquer method.

desi_coadd_spectra --help
usage: Coadd all spectra per target, and optionally resample on linear or logarithmic wavelength grid
       [-h] [-i INFILE] [-o OUTFILE] [--nsig NSIG] [--lin-step LIN_STEP]
       [--log10-step LOG10_STEP] [--wave-min WAVE_MIN] [--wave-max WAVE_MAX]
       [--fast]

optional arguments:
  -h, --help            show this help message and exit
  -i INFILE, --infile INFILE
                        input spectra file
  -o OUTFILE, --outfile OUTFILE
                        output spectra file
  --nsig NSIG           nsigma rejection threshold for cosmic rays
  --lin-step LIN_STEP   resampling to single linear wave array of given step
                        in A
  --log10-step LOG10_STEP
                        resampling to single log10 wave array of given step in
                        units of log10
  --wave-min WAVE_MIN   specify the min wavelength in A (default is the min
                        wavelength in the input spectra), used only with
                        option --lin-step or --log10-step
  --wave-max WAVE_MAX   specify the max wavelength in A (default is the max
                        wavelength in the input spectra, approximate), used
                        only with option --lin-step or --log10-step)
  --fast                fast resampling, at the cost of correlated pixels and
                        no resolution matrix (used only with option --lin-step
                        or --log10-step)