Add a load_graph attribute to BasePartitioner

[csebastiao]

Add an attribute load_graph=None where we can load a networkX graph (or a graphml) into the BasePartitioner. As of now, can only either download it straight from OSM, or need to already have the graph inside the Config.GRAPH_DIR folder with the name {city_name}.graphml who need to have already some metrics computed on.

Necessary for any graph not coming from a simple Nominatim call.

[csebastiao]

Since the graph might not "work" as you could pass any networkX graph, need to test if the graph is following the necessary format to use BasePartitioner.