Move astronomy schemas to asdf-astropy

[braingram]

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)

[braingram]

Bringing this up again. The recent issues with magunits as units to columns (see asdf-format/asdf#2000) has again highlighted the complications caused by having these schemas in the asdf-standard package. We're currently relying on some brittle connections between the serialization of magunit matching unit (which it is not) and ideally we would define the acceptable tags (unit, magunit, etc) in the column schema. However that would create references (more accurately tag wildcards) for astropy tags in core schemas.

[braingram]

To add another argument the current column schema requires "name":

asdf-standard/resources/stable/schemas/stsci.edu/asdf/table/column-1.2.0.yaml

Line 47 in daff9d5

required: [name, data]

However it's perfectly valid to have an astropy Column with no name. However this fails to save since the converter produces a name of None. That's certainly an asdf-astropy bug but what is asdf-astropy supposed to do here? It seems like the astropy column implementation is not a close enough match to the column schema/tag so I think we need a new astropy column tag/schema.