Make ASDF extension robust to missing asdf-astropy

[larrybradley]

This is a followup to #2211. Initial ASDF converters were added in #2211, but with asdf-astropy as an optional dependency. Previously, attempting to serialize a CircularAperture or AiryDiskPSF to ASDF without asdf-astropy installed raised an AsdfSerializationError with a long traceback.

This PR fixes that by always registering both converters. CircularApertureConverter works fully with only asdf. AiryDiskPSFConverter falls back to a plain asdf.extension.Converter base class when asdf-astropy is absent, and its to_yaml_tree/from_yaml_tree methods raise a clear ImportError pointing the user to pip install asdf-astropy instead of a deep traceback.

[larrybradley]

Pinging @braingram as a potential reviewer too.

[braingram]

This seems like a complicated distinction to make. Is there much benefit in supporting an environment that supports serializing some objects when asdf is installed and not asdf-astropy (which essentially only depends on asdf and astropy)? For example is it possible to construct a CircularAperture with quantities? Those wouldn't be supported without asdf-astropy and wouldn't trigger the custom error message added here.

If the goal is to provide a custom message if asdf is used but asdf-astropy is not installed perhaps it would be easier to register a extension that supports all photutils tags and photutils types (that have asdf support) and displays the message on read/write?

[larrybradley]

@braingram Many thanks for the review! CircularAperture can't be constructed with Quantities, but SkyCircularAperture (which I assume will be coming) requires Quantities (and SkyCoord).

The goal was provide a custom/friendly error message. My solution also doesn't scale -- and @nden is adding more converters (to cover all aperture and psf models). sunpy also has asdf-astropy as an optional dependency. I'll take a look to see if they have added any custom error messages or anything similar. I might be overthinking this...