astroplan and exposure time calculator (ETC) functionality

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.: https://github.com/astropy/astroplan/pull/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.

Start by reading the proposed astroplan implementation in pull request #420 and the gunagala documentation linked in the issue. Compare the scope of an ETC inside astroplan with an independent affiliated package, including the possible image-simulation and spectroscopy extensions. Done means reaching and documenting a community decision about the package location and intended scope.

Written by the indexing model from the issue text.