Create dedicated properties and shapes for specific data identifiers (doi, pmid)

[MFSY]

Since multiple identifiers are needed (doi, pubmed id,...) it may be useful to create dedicated properties for them just like schema.org does. For example an "isbn" property is provided as an identifier sub-property.

The goal is to be able to assert that an entity (an article) can have a doi and/or pubmedID as identifiers in the following way:

 {
     "path": "nsg:doi",
     "name": "DOI",
     "description": "A doi identifier",
     "node": "{{base}}/schemas/neurosciencegraph/commons/identifier/v0.1.0/shapes/DOIShape",
     "maxCount": 1
}, {
     "path": "nsg:pmid",
     "name": "PMID",
     "description": "A PubMed identifier",
     "node": "{{base}}/schemas/neurosciencegraph/commons/identifier/v0.1.0/shapes/PMIDShape",
     "maxCount": 1
}

The shapes {{base}}/schemas/neurosciencegraph/commons/identifier/v0.1.0/shapes/DOIShape and {{base}}/schemas/neurosciencegraph/commons/identifier/v0.1.0/shapes/PMIDShape can extends the IdentifierShape.

[pafonta]

Small comment from #43 (comment):
The URL could be the only way to identify some literature resources which need to be annotated. It might therefore also be considered as an 'identifier'.