Compute set of edges that match generic location reference

Create a set of methods or class that accepts a generic location reference (not tied to OSMLR) and returns a set of edges. This may (?) assume the location reference points are actual nodes / intersections and that the edge walking code within valhalla_associate_edges may be sufficient.

Code that does this within valhalla_associate_edges is part of the edge_association class. This uses loki to search for edges within the tolerance, but if the location reference points are constrained to being nodes/intersections the simpler find_nearby_nodes method could be used.

Note that there is code within baldr merge.cc to create "segments" that could potentially be used to create test location references to use when testing the methods.

Feels like this class belongs in baldr.