bugfix/label_sliced_way_ends

[thomas-fred]

osm_to_pq.py clips OSM .pbf files of ways to a bounding box and converts the ways to GeoDataFrames of edges, split at junctions. It also tries to label which edges have termini (starts or ends) resulting from being clipped.

An OSM way may be split into several segments, where nodes from other ways intersect the way in question. Previously, it was assumed that any segment ending in a node that wasn't 'shared', i.e. degree >= 2, must have been clipped. In fact, this also includes dead ends and so erroneously labelled many edges as having been clipped.

This PR reworks the script, using an index of all nodes (not just shared ones) to check if a segment end is new (and therefore resulting from a clip operation). It also hopefully improves the readability (although it's still a bit hard to follow).

Fixing this is a step towards network component labeling at scale, i.e. on sections of networks, before joining the sections on the clipped boundaries later.

[thomas-fred]

Should close #114