Removed exception in add_field for at_grid. [skip ci]
Removed exception in add_field for at_grid. [skip ci]

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