Allow for importing edge length from osm.pbf input

[zneedell]

For BEAM we use osmnx to clean up an OSM network before reading it into R5. This simplifies the network by removing some intermediate nodes and not keeping link geometries. So, estimating link length based on the distance between start and end node can lead to a big underestimate of total length.

Here:

r5/src/main/java/com/conveyal/r5/streets/StreetLayer.java

Line 1058 in 923bb13

int edgeLengthMillimeters = getEdgeLengthMillimeters(nodes);

the edge length is calculated based on straight line distance. We should update it to first look for a "length" tag in the edge and only calculate the length if that doesn't exist.