Add a way to enumerate edges connected to `GraphVertex`

[Xrayez]

This allows to traverse edges over of vertices:

for edge in v.get_edges():
    print(edge.a, " ", edge.b)

for edge in v.get_incoming_edges():
    # `b` is predecessor of `a`
    print(edge.a, " ", edge.b)

for edge in v.get_outgoing_edges():
    # `b` is successor of `a`
    print(edge.a, " ", edge.b)

@sairam4123 https://github.com/goostengine/goost/pull/172#issuecomment-1044409628

[sairam4123]

Thanks! That's helpful!