Ensure "processing model" is included as a registry entry option

[dlongley]

          @wes-smith as you know we'll need to update this with the new table layout (string, url scheme, typed-literal, etc.). But we also need to include that a registry entry indicates a "processing model" and we need to indicate that there is a "default" processing model that every entry uses unless otherwise specified and that refers to the algorithms in this spec (either present or not yet added).  The processing model indicates how context-based, auto-generated term IDs are generated, what the core / generic type-encoders are (e.g., for date compression) and so on.

Originally posted by @dlongley in #28 (comment)