time schema is inconsistent with asdf-astropy

[braingram]

This has been hidden by and is related to #345

asdf-astropy serializes astropy.time.Time values in a way that is inconsistent with the time schema. One example is that the schema lists allowed base_formats:

    enum:
      - iso
      - yday
      - byear
      - jyear
      - decimalyear
      - jd
      - mjd
      - gps
      - unix
      - cxcsec

which are incomplete compared to the base_format values produced by asdf-astropy (which can contain any astropy.time.Time format). For example, serializing

astropy.time.Time(730120.0003703703, format="plot_date")

produces

t: !time/time-1.1.0 {base_format: plot_date, value: '2000-01-01T00:00:32.000'}

Given the assumed intended behavior of base_format (to store the astropy.time.Time format) this looks more like a schema bug than an issue with the asdf-astropy implementation.

The base_format is one (but possibly not the only) error with the time schema that was hidden by the ndarray schema issue.