Add unit tests for GraphFields.
Add unit tests for GraphFields.

landlab/landlab,csherwood-usgs/landlab,Carralex/landlab,Carralex/landlab,csherwood-usgs/landlab,landlab/landlab,amandersillinois/landlab,RondaStrauch/landlab,cmshobe/landlab,cmshobe/landlab,cmshobe/landlab,Carralex/landlab,landlab/landlab,RondaStrauch/landlab,amandersillinois/landlab,RondaStrauch/landlab