Refactor split_long_edges

[barneydobson]

The function is not pretty and inefficient.

Suggestions:

This will be much more efficient if done on the GeoDataFrame representation of the graph. First, use ox.utils_graph.graph_to_gdfs to convert to a gdf, then do the operations and once done, use ox.utils_graph.graph_from_gdfs to convert it to a graph. You can also use shapely.segmentize.

[barneydobson]

Additionally current implementation can create edges with the same id as nodes, which SWMM doesn't allow (fixed in #83 by calling assign_id again - but it isn't very satisfying).

[barneydobson]

Fixed with #142