Added doctests for link unit vector methods.
Added doctests for link unit vector methods.

landlab/landlab,Carralex/landlab,csherwood-usgs/landlab,amandersillinois/landlab,amandersillinois/landlab,csherwood-usgs/landlab,cmshobe/landlab,cmshobe/landlab,RondaStrauch/landlab,SiccarPoint/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,ManuSchmi88/landlab,SiccarPoint/landlab,Carralex/landlab,landlab/landlab,laijingtao/landlab,Carralex/landlab,RondaStrauch/landlab,laijingtao/landlab,cmshobe/landlab,ManuSchmi88/landlab,landlab/landlab