Faster vertex()

Hello,

I was adding a lot of edges on one of my graph, using vertex() to get vertices and my code was kind of slow.

I did some benchmarking and found that a lot of time was spent inside the vertex() function (used by my code and add_edge()), especialy by calling get_vertex_filter and set_vertex_filter.

Since I wasen't using any filter, I just simplfied vertex() to

def vertex(self, i, use_index=True):
    return libcore.get_vertex(weakref.ref(self), int(i))

This is an ugly hack, but I got x4 speedup.

Would be possible to add an option to disable any *_vertex_filter call ? Eg. if the user didn't set a filter or with an option.

I can provide a merge request if needed :)