Add (optional) peak metrics from skycorr-PCA to the sky*.fits files

[rongpu]

This PR adds the option to add the sky line peak metrics that can be useful diagnostics –– see the example below. Specifically, it adds these 6 FITS HDUs: PEAK_DW_BEFORE, PEAK_DLSF_BEFORE, PEAK_CHI2PDF_BEFORE, PEAK_DW_AFTER, PEAK_DLSF_AFTER, PEAK_CHI2PDF_AFTER.

The addition HDUs only increases the file size by ~10%, and since the sky files are already small (~5 MB), the size increase would be quite negligible. So we could also consider adding these HDUs as a default (right now it is disabled by default). The other advantage of having these HDUs is that we can use them to directly compute the skycorr-PCA templates –– the current procedure requires rerunning desi_proc which is very cumbersome.

I should note that while I have tested the new code by running desi_compute_sky, I have not done extensive testing with the full pipeline.

[rongpu]

Correction: the additional HDUs are not sufficient to compute skycorr-PCA templates. To do that, we would also need to save the peak wavelength and the DWAVE/DLSF errors.