Look into using nhdplusTools helpers instead of traverse_nhd()

[lekoenig]

I'm wondering if the code to build the NHM - NHDPlusv2 crosswalk table could be made simpler and thus more readable by replacing traverse_nhd() with some helpers from nhdplusTools. Due to idiosyncrasies in the NHM v1.0 network this may not necessarily be a better solution to what we've got, but probably worth considering.