allow for optional import of intersection data (eg official data, or externally processed)

[carlhiggs]

Currently intersections are represented using the consolidate intersections function of OSMnx, however as per #138 this doesn't necessarily work well in all settings. In brief, a tolerance parameter (metres) is used for simplifying the network, however this is designed more for use in localised areas without variation in network structure. In contrast, for whole city analysis with broad variation in network topology as well as in complex features like round abouts, it may be desirable to use an alternative data source or externally processed (cleaned) data to represent intersections more accurately.

This is simple to implement, by allowing users to specify a custom intersection dataset, and if this is configured it will be imported; otherwise, the default will be to derive a dataset of intersections using OpenStreetMap.