Removed old commented code on UnityReader
Renamed argument "date" to "time" in EntityVertex
Changed a comment on Edge
Added vertex (Activity, Entity, Agent) reader on PROVReader
Refactored Vertex and Edge to extend GraphObject
GraphObject created to manage Attribute Map used by both Vertex and Edge
Added new Function in XMLReader to add an edge (another overload of AddEdge) for attributes
Added PROV-XML edge-related functions on PROVReader (Missing complex ones such as Derivation and Delegation). However, edge calls are not implemented yet, neither the section for adding the edge in the graph
Commented the sort agent by name in temporal layout due to problems in the comparator