avdata_import() enhancements

[mtmorgan]

• Handle 'Description' fields

  • avdata() would parse __DESCRIPTION_<key> attributes to a 'Description' column
  • avdata_import() would do the reverse

• implement avdata_delete() to remove particular keys.

• round trip a vector-of-links. Currently avdata_import(avdata()) converts to a JSON text representation of vector of strings ["gs://...", "gs://...", ...].

[smgogarten]

+1 to handling Description fields; I would like to use this feature if it were available