io.skymap.read_skymap(): no_hdu parameter

[Theodlz]

We avoided memory issues in SkyPortal (in production) by dropping the hdu variable (and others) created by gwemopt when reading a skymap before calling the scheduler, however just creating the variable can require some amount of memory and create issues, so this PR simply adds an argument to avoid creating said variable altogether. The default is to still create it (to not impact any existing workflows), but one can explicitly avoid it by setting that new optional flag to True.

I also rearranged a few things in that function + fixed one line that looked suspicious to me (where the skymap_schedule variable was set using the skymap_raster and not the skymap_raster_schedule).

[mcoughlin]

Thanks @Theodlz

[Theodlz]

@mcoughlin there are a few other variables: skymap_raster, skymap_raster_schedule, center that are only useful if:

• skymap_raster: computing efficiency
• center: plotting
• skymap_raster_schedule: reading a galaxy catalog (only when calling gwemopt's run.py)

so I think I can make this even more modular based on what's needed downstream, let's not merge it yet

[Theodlz]

Ok just made that change, I'll wrap it up tomorrow and make a release this week ideally