MOD API should also retrieve scheme registries (aka terminology collections)

[nichtich]

Add getRegistries to mod-api-provider.js to retrieve "terminology collections" (aka JSKOS Scheme Registries). Use data from endpoint https://terminology.services.base4nfdi.de/api-gateway/ for testing. Extend documentation. Two modes of operation:

• get full list of scheme registries via /collections/ (e.g. https://terminology.services.base4nfdi.de/api-gateway/collections/)
• get individual scheme[s] registry via its URI[s] (not implemented yet, requires Include URIs of collection entities ts4nfdi/api-gateway#150)

Temporary workaround for the second may be getting all collections and filtering in the client (slow).

The returned data is not strictly defined, as requested here but a custom format to be mapped to JSKOS Scheme Registries. Expected output:

{
  "type": ["http://www.w3.org/ns/dcat#Catalog"],
  "types": [ { "uri": "http://www.w3.org/2004/02/skos/core#ConceptScheme" } ],
  "prefLabel": { "en": "...label..." },
  "scopeNote": { "en": [ "...description..." ] },
  "schemes": [ 
    { "uri": "...", "prefLabel": { "und": "...label..." } },
    ... 
  ],
  "creator": [ { "uri": "..." } ],
  "contributor": [ { "uri": "..." }, ... ]
}

Notes:

• creator and collaborators in current data cannot be mapped yet, because they are not URIs. Give a warning instead.
• Filter out terminologies with uri not being a valid URI and emit a warning