astroplan and exposure time calculator (ETC) functionality

[lspitler]

This is more of a prompt for discussion on how the astroplan community would like to incorporate exposure time calculator (ETC) functionality into the astropy etc ecosystem.

One way to help frame the discussion is to consider two (of many?) possible options:

1. build the capability into astroplan directly, e.g.: Adding an exptime calculator, Telescope class, and sky model #420
2. spin it off into its own module, which allows for more functionality to be developed, image simulations, spectroscopy mode, etc. See the gunagala module for an example of this idea: https://gunagala.readthedocs.io/en/develop/

What do people think of this? Is astroplan the right place for an ETC? Or is an independent, astropy-affiliated module like gunagala the place to spend effort? Or does the community want both?

Note, I'm leading the project ( https://huntsman.space/ ) that in part inspired @AnthonyHorton to create gunagala. I am considering whether to seek FTE resources to bring gunagala up to the level of an astropy affiliated package.

Thanks in advance for sharing any thoughts or ideas you might have.