Genre aliasing

[NicolasCARPi]

          Following @SteffenBrinckmann comment on #103, `@type` MUST be a single string value while additional types should go to `genre`.

I understand the point of limiting variances.
However, @type is rather central in any RDF/JSON-LD reasoning, validation and framing operation.
A workaround would be to alias genre as @type ìf required by an interpreter

{
  "@context": [
    "https://w3id.org/ro/crate/1.1/context",
    {
      "genre": "@type"
    },
    "type": "schema:Dataset",
    "genre": "ex:CustomType"
  ]
}

In this case it would be helpful to allow genre to by an array of IRIs. This would allow the TheELNConsortium to specify a list of common types while vendors can add more specific subtypes to persist internal classification.

{
  "@context": [
    "https://w3id.org/ro/crate/1.1/context",
    {
      "eln": "https://w3id.org/TheELNConsortium/",
      "elabftw": "...",
      "osw": "https://opensemantic.world/id/"
    }
  ],
  "genre": [
    "eln:Experiment",
    "osw:Category-3AOSW0e7fab2262fb4427ad0fa454bc868a0d"
  ]
}

Otherwise additionalType IRIs may be the right location for those IRIs

Originally posted by @simontaurus in #69 (comment)