post_process shapes: calculate missing shape_dist_traveled

[fpurcell]

RTD's gtfs does not have the (optional) shape_dist_traveled field populated.
Let's run thru each shape, and calculate those values.

Thus this:
shape_id,shape_pt_lat,shape_pt_lon,shape_pt_sequence
1160070,39.857378,-105.054026,1
1160070,39.857013,-105.053717,2
1160070,39.857013,-105.053718,3
...

Becomes this:
shape_id,shape_pt_lat,shape_pt_lon,shape_pt_sequence,shape_dist_traveled
1160070,39.857378,-105.054026,1,0.0
1160070,39.857013,-105.053717,2,55.0
1160070,39.857013,-105.053718,3,111.0
...

This will help with https://github.com/OpenTransitTools/trafficdb