Refactor DatasetService

[srosset81]

The current DatasetService interacts directly with the Fuseki API to create and delete datasets. We will keep this service, but we will create another one which will interact with the NextGraph API and take into account its particularities.

• Rename DatasetService to FusekiDatasetService
• Create new NgDatasetService
• Actions to rewrite with NextGraph:
  • create
  • delete
  • list: Available also with NextGraph.
  • exist: NextGraph throws an error if you try to open an unavailable dataset
  • backup: Do it manually ?
• On PodService, ensure the dataset is correctly created after auth, and that the correct informations are saved on the "settings" dataset (see Initiate the NextGraph server #1262)

Information from Niko: For the moment, the NextGraph library only allow to create private stores. "Protected" stores would be better, as data can be published. In the future, the library will allow to choose the type of store we want to create.