Update core for ASDF standard 1.6.0

[braingram]

Removes core/column and core/table (replacing them with table/column and table/table).
Removes all non-core tags from the core manifest, putting them in a astronomy manifest.
Removes subclass_metadata from 1.6.0.
Update asdf-schema metaschema version to 1.1.0 to pick up ndarray-1.1.0 datatypes.

[eslavich]

Since we're modifying this anyway, should we convert it to an asdf://asdf-format.org URI?

[braingram]

I'm all for the change but would rather change (or attempt to change) all the tags at once. Perhaps that's a good target for 1.7.0?

[eslavich]

Ah bummer. Such is life :)

[nden]

LGTM.
One question, for my understanding - Is the effect of removing the "astronomy" tags from version_map-1.6.0 only that they are not included in the 1.6.0 standard? Does it affect in any way the ability to version them, e.g. read a file with an old astronomy tag and write it out using the current version?

[braingram]

Thanks for the review!

The version maps have been largely replaced by the extension manifests. They are currently only used to determine the YAML (1.1) and file format (1.0) versions. asdf-format/asdf#1745 is work towards removing them (by instead defining these versions as constants). The tags listed in the version maps are unused in asdf (their usage was removed when the type index was removed in asdf 3.0).

The tag selection for reading/writing is now entirely defined by the extensions (and manifests). To use core/column-1.0.0 as an example, let's say a file was written with asdf-standard 1.5 (the current latest 'stable' version) and that file contains a core/column-1.0.0 tag. When the file is read, asdf will look for an extension that supports core/column-1.0.0, it will find the core-1.5.0 extension (provided by asdf-astropy and defined from the 1.5.0 manifest which defines core/column-1.0.0). The tag will be read in as an astropy.table.column.Column. If this file is then rewritten asdf will look for an extension that support astropy.Column and what occurs depends on the asdf standard version provided to write_to. By default the latest stable version (1.5.0) will be used. asdf will find the core-1.5.0 extension (which requires asdf-standard version 1.5.0) and write out the Column using the core/column-1.0.0 tag. If instead, asdf standard 1.6.0 is defined, the core-1.5.0 extension will be skipped (the standard requirement isn't met) and instead asdf will use the new astronomy manifest (with support for this extension being added to asdf-astropy in this PR: astropy/asdf-astropy#219) and the Column will be written with the new table/column-1.0.0 tag. Let me know if more details would be helpful.