[BUG] DOIs and other identifiers of Publications in RO-Crate

[floWetzels]

The current RO-Crate export does not follow the ISA-RO-Crate Profile correctly.
Properties like PubMedIDs or DOIs of a ScholarlyArticle should be mapped into the schema property identifier, see details in the corresponding Issue.
Currently, they still are put into sameAs, which can be seen in the RO-Crate of the Facultative-CAM-in-Talinum ARC.
Here's an example snippet:

{
      "@id": "10.1104/pp.15.01076",
      "@type": "ScholarlyArticle",
      "author": [
        {
          "@id": "_:b0"
        },
        {
          "@id": "_:b1"
        },
        {
          "@id": "_:b2"
        },
        {
          "@id": "_:b3"
        },
        {
          "@id": "_:b4"
        }
      ],
      "creativeWorkStatus": {
        "@id": "#UserTerm_Published"
      },
      "headline": "Reversible Burst of Transcriptional Changes during Induction of Crassulacean Acid Metabolism in Talinum triangulare.",
      "sameAs": "10.1104/pp.15.01076",
      "url": "26530316"
    }