add skos-xl model

[syphax-bouazzouni]

Requirement

• [x] Add the skosxl:Label class to the model

• [x] Add the endpoint "/skos_xl_label/:id" to show the skosxl:Label model by :id

• [x] Add tests

  Deployment

• [x] testportal

• [x] stageportal

• [x] agroportal

• [x] bioportal

[syphax-bouazzouni]

Done