Move astronomy schemas to asdf-astropy #482
Description
Many of these are very specific to astropy. Moving them to asdf-astropy would:
- simplify the core schemas
- make it easier to maintain these schemas (see fix time-1.4.0 #481)
- make it easier to maintain asdf-astropy (so that asdf-standard releases aren't needed for asdf-astropy and it can make less version checks against asdf-standard and conditionally register extensions)