Added missing imports in doctests.
Added missing imports in doctests.

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