more flexible line-fitting++

[moustakas]

[WIP] This PR adds:

• Fixes for cascades;
• Adds MgII and several other nebular lines ([Ne V], [Ne III], [SIII], etc.);
• Allows the forbidden and Balmer line-widths and velocity centroids to be different, which can be separate from MgII, which is yielding some very interesting (useful!) results;
• By default, iterates the line-fitting in order to support the preceding step.

There is a computational penalty to the code changes in this PR but on-balance they are in the right direction for numerous applications, including data-driven and target-specific improvements of the redrock templates.

[moustakas]

Tested with three deep tiles of each targeting flavor, 80607 (QSO+LRG), 80608 (ELG), and 80613 (BGS+MWS), totaling about 10,000 spectra and (separately) broadband photometry fitted.

Bone-simple visualizations are here--
https://data.desi.lbl.gov/desi/users/ioannis/fastspecfit/cascades/

and merged fitting results are here--
/global/cfs/cdirs/desi/spectro/fastspecfit/cascades/tiles/merged
https://data.desi.lbl.gov/desi/spectro/fastspecfit/cascades/tiles/merged/