Support HgCd and other multi and single element sources

[Updated title and description, as work reflects a bigger change than the original description implied]

The current reference lines construction code consists of a set of single-element linelists (Hg.txt, Ar.txt, Xe.txt, etc) as well as a linelist for the sky (skyLines.txt). For single-element lamps (Hg, Ar) the appropriate file is read, while for lamps that contain a mix of elements (eg HgAr) the Hg.txt and Ar.txt files are read and merged.

This is not entirely reflective of the real situation, where due to internal physics, the lamps that contain multiple elements may result in emission spectra where the lines for a given species may be shifted slightly compared to that of the single-element lamp case. So dedicate linelists for each lamp or light source is required.

Please add support for dedicated linelists for HgCd and HgAr lamps (the former is currently missing), while retaining support for single-element lamps.

Update reference line construction code to handle multiple lamps being illuminated (be they consisting of multiple elements or a single element).

For the HgCd lamp, update the ops_pfs code such that the lamp is correctly identified when the FITS keyword W_AITHGC is True in the raw exposure data.