Add an example where a path includes constant modulators

[krlawrence]

Add an example such as

gremlin> g.V().has('code','YPO').outE().inV().outE().inV().simplePath().path().by(constant(0)).by('dist')
==>[0,188,0,55,0]
gremlin> g.V().has('code','YPO').outE().inV().outE().inV().simplePath().path().by(constant(0)).by('dist').sum(local)
==>243