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.

[cbueth]

Thanks @csebastiao for creating this issue. As it has come back up now we will reactivate it, adding the ability to import a graph independently of the automatic downloading. Has there been any updates from your side, @csebastiao?
When importing there are preprocessing steps that would need to be done to the graph and we might save a prepared copy of it. This way the expensive and static preprocessing (i.e. population distribution) is independent of the partitioning where a user might like to change parameters and experiment with the outcomes.
If someone might like to work on it, please let us know here in a comment. I'm happy to help with the design decisions and guidance.