Fixed doctests.
Fixed doctests.

Import tested functions into doctests to fix `NameError` errors.

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