newexp-desi --flavor bgs and bright

[sbailey]

Once #70 (add BGS-like templates) is done, add newexp-desi --flavor bgs and bright options.

--flavor bgs would simulate just Bright Galaxy Survey targets, while bright would do a mix of BGS and MWS targets based upon the target densities in $DESIMODEL/data/targets/targets.dat .

Use exptime_bright from desimodel.io.load_params() for exposure time for flavors mws, bgs, and bright.

[moustakas]

I note that that file (targets.dat) doesn't contain any information about MWS, and the BGS numbers are woefully out of date / incorrect.

A related question: will MWS be applying color cuts to select particular subpopulations?

[sbailey]

Placeholder numbers for MWS, BGS have been added to desimodel and PR #92 fixed a bug for newexp-desi --flavor bright. Closing ticket.