Computing Path length

[mikejohnson51]

Currently, path length is computed based on the geometry of a NHDPlus, refactored, or aggregated flowpath.

When we use a cat-->nex topology, this requires water to be "flown" around.

I want to test an alternative path metric that computes the lengths of a flow path from catchment inlet to the downstream nexus - this is an effective "virtual path" for routing