Added doctest for midpoint_of_link.
Added doctest for midpoint_of_link.

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