Encode directionality into FLOW counts

[cczhu]

Currently, prj_volume.uoft_centreline_volumes_output uses a grid angle system to delineate traffic direction, where +1 roughly reflects north and eastbound and -1 roughly reflects south and westbound. This is difficult to parse back into road directions eg. on the directional centreline for routing purposes. We need to replace this system with one that allows us to easily determine which direction along a road each count represents, which would let us calculate traffic upstream and downstream from a point.

One possible solution would be to associate each count with the directional centreline. It might also make sense to be able to map counts to the system we use to calculate network distances. @radumas would have a much better sense of an ideal solution.

Regardless of the solution, it could be done by modifying the arterycode mapping process developed by @aharpalaniTO et al., documented here.