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

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