need to be able to simulate (noisy) DA and DB white dwarf spectra

[moustakas]

The ability to generate subtype DA and DB white dwarf templates was added in #200 but newexp-desi, pixsim-desi, and quickgen do not handle this "flavor" of target. We should fix this with the new refactor.

This issue is kind of related to #203 and #97.

[moustakas]

Simulating both DA and DB WDs is now a standard part of the spectra generated with desitarget/bin/select_mock_targets (as long as the yaml configuration file points to an appropriate WD mock, see, e.g., here).

In addition, smaller-scale simulations of WDs can be done trivially by calling desisim/bin/quickspectra (by way of a simple example, see this notebook, which simulates 1000 stand-alone BGS spectra).

Members of the MWS Working Group should reopen this issue if they have more specific needs not covered here.