Deprecate Nested RecordSets in favor of repeated subField

[benjelloun]

The Croissant Spec allows nesting RecordSets inside RecordSets, by using a field with dataType="cr:RecordSet"

https://docs.mlcommons.org/croissant/docs/croissant-spec.html#nested-records

This mechanism has not been used much, is not supported in the mlcroissant library, and adds unneeded complexity.

Instead, we propose using the existing subField mechanism, and specifying repeated=true to represent multiple records.

Here is an example based on the one in the above documentation:

{
  "@type": "cr:RecordSet",
  "@id": "movies_with_ratings",
  "key": { "@id": "movies_with_ratings/movie_id" },
  "field": [
    {
      "@type": "cr:Field",
      "@id": "movies_with_ratings/movie_id",
      "source": { "@id": "movies/movie_id" }
      "references" :  { "@id": "ratings/movie_id" }
    },
    {
      "@type": "cr:Field",
      "@id": "movies_with_ratings/movie_title",
      "source": { "@id": "movies/title" }
    },
    {
      "@type": "cr:Field",
      "@id": "movies_with_ratings/ratings",
      "repeated": "true",
      "subField": [
        {
          "@type": "cr:Field",
          "@id": "movies_with_ratings/ratings/user_id",
          "source": { "@id": "ratings/user_id" }
        },
        {
          "@type": "cr:Field",
          "@id": "movies_with_ratings/ratings/rating",
          "source": { "@id": "ratings/rating" }
        },
        {
          "@type": "cr:Field",
          "@id": "movies_with_ratings/ratings/timestamp",
          "source": { "@id": "ratings/timestamp" }
        }
      ]
    }
  ]
}

Note that using a repeated field with subFields also enables us to get rid of the cumbersome "parentField" property in the previous syntax. Instead, the join with the underlying ratings table is specified on the "movie_id" property.