Test the `update_cost` function

[PatrickOHara]

Is your feature request related to a problem? Please describe.

The update cost function take the air quality predictions (hexgrid), does an spatial intersection with the road network then applies a cost function to each edge.

It needs to be tested properly.

Describe the solution you'd like

• [ ] Test setting fill_edge_geometry to True does something sensible.
• [ ] Test that the cost function is calculated properly. Generalise to allow different cost functions to be applied.
• [ ] Create a fake road network and fake grid for testing geospatial queries.

Describe alternatives you've considered

We run the tests on the osmnx graph itself, but its difficult to verify correct results on a real graph

Additional context

[PatrickOHara]

This function is superceded by the new RoadQuery object created in #34