Refactor create_model to iterate over registered functions that take graphs and give graphs

[barneydobson]

Preferred function format will follow the format:

@register_graphfcn
def generic_function(G, a, b, **kwargs):

    # create G_ from G, a, b
    return G_

And create_model will primarily call:

for function in list_of_functions 
    G = function(G, **parameters, **addresses)
    if verbose:
        graph_to_geojson(G, function.__name__ + .geojson)
save_graph(G, function.__name__ + .json)