[API] Duplicated functionality of graph

Created by: PhilipVinc

From AbstractGraph:

    @property
    @abc.abstractmethod
    def n_nodes(self):
        r"""int: The number of vertices in the graph"""
        raise NotImplementedError

    @property
    def n_sites(self):
        r"""int: The number of vertices in the graph"""
        return self.n_nodes

    @property
    def n_vertices(self):
        r"""int: The number of vertices in the graph"""
        return self.n_nodes

I'd argye to keep only n_vertices, because it's the same terminology of networkx.

• We are missing a function returning an iterator over vertices (they are not necessarily ordered, though usually so...)

• The networkX api has edges as a property. For us it's a function. The only reason for this is because we want to be able to call edges(color=True), but that is only supported on some graphs. I'd therefore argue to make all fields in graph properties, and graphs that support colors should just define a new method (colored_edges) that is an additional property.