Identify what should go into `astar_utils`

[teutoburg]

Some functionality was already moved to AstarVienna/astar-utils.

We should decide what else we want there. Some points to consider:

1. I'd like to keep the dependencies of astar-utils to a minimum. Specifically:
   a. Try to avoid astropy, matplotlib and synphot, as those add a huge overhead.
   b. Any functionality that uses only the standard library can easily be moved to astar-utils (if it's a general utility).
2. On a related note, I think it's best to avoid any network/download-related functionality there. After talking to @astronomyk, we considered a separate package astar-webutils for that purpose, if it's worth outsourcing this at all.

In general, I might look though the utils.py modules of our various packages to see if there's any existing overlap or potential for re-use in another of our packages.

This issue is more a way to keep track of this in the scope of the ScopeSim v1.0 project, rather than an immanent action...