Use the /data RDF store to avoid having to recreate Environment, Workspace and Artifact in Light308 demo

[asorici]

Context

In the current demo, testing the functionality involves deleting the contents of the ./data folder to ensure new creation of an environment, workspace and light308 artifact every time a test is carried out.

Part of the reason is discussed in Issue #1 (see proper URI configuration).
The other reason is that current flow requires the in-memory population of the HypermediaArtifactRegistry (see the call in setupOperations() in HypermediaAritfact ), as well as the registration of the Interaction Affordances (the actions - state and color) with an instance of the artifact.

The /data RDF store only contains the representation of the artifact (which can be served upon request), but the in-memory instance needs to be recreated as well.

TODO

1. Option 1: Modify the startup of an Yggdrasil platform to check for existing environments, workspaces and artifacts.
   If an artifact is detected, it should be automatically be instantiated. NOTE that this will work for our light308 demo, because our artifact does not require instantiation parameters (otherwise those would need to be persisted as well, somewhere in the artifact state representation).
   The relevant call to be made is to the CREATE_ARTIFACT branch of the CartagoVerticle -- i.e. an internal vertx request needs to be made to the CartagoVerticle to instantiate the artifact (e.g. named light308) that has been identified as persisted in the /data RdfStore.

2. Option 2: an easier option is, on every startup, to just keep the environment and workspace instance, but clear all artifact instances (since these ones have internal states which would need to be persisted as well - and this is not always the case with the ThingDescription based representation).

This option implies an access to the RdfStore, a retrieval of all instances of HypermediaArtifacts and a removal of all the RDF triples related to those instances.
To properly DEBUG that the clearing works, a helper method should be created to serialize the contents of the RDF Store into a Turtle file for easy visual inspection that NO unwanted triples related to an artifact instance remain after a clearing operation.