tickets/SP-2754: add no_sky=True to ModelObservatory wherever possible

[ehneilsen]

In many cases, schedview creates instances of ModelObservatory that do not need sky brightness, and this change adds the no_sky=True flag when these are instantiated.

In some cases, however, the sky brightness is used, in particular when maps of sky brightness are created (as in the scheduler snapshot dashboard) and when the values of basis functions are computed.

Even for these cases, it is still possible to use schedview without local copies of sky brightness pre data files, if:

• the SIMS_SKYBRIGHTNESS_DATA environment variable is set to a URL under which sky brightness files can be found, for example https://s3df.slac.stanford.edu/groups/rubin/static/sim-data/sims_skybrightness_pre/h5_2023_09_12_small/
• the lsst.resources path can be imported

In this case, sky brightness data will be downloaded from the USDF as needed. (This is how the schedview snapshot dashboard does it.) It is, however, much slower than using local files, particularly from IP addresses that are not at SLAC.

[rhiannonlynne]

I tested this with the night summary locally and it works! (for the visit maps, is the use-case I was interested in, in particular).