Port new instances and types

[filippomc]

Now we are able to directly define instances on the geppetto model from the backend. New instances are coming inside variables a new container called "worlds". Old variables container is being deprecated from now on.

Features to implement:

• Regenerate model
• GEPPETTO.ModelFactory.allPaths should return the paths for all the worlds. The path will be [WORLDID].[VARIABLEORINSTANCEPATH] -> GEPPETTO.ModelFactory.fetchAllPotentialInstancePaths
• Instances.getInstance must work with the new structure -> fill GEPPETTO.ModelFactory.allPathsIndexing
• For instances, values should be easily accessible. Say we have an instance myworld.myInstance with a value of type JSON {"a": 1}. Instances.getInstance("myworld.myInstance").value.a should return 1

[filippomc]

@tarelli @gidili