Changed default for map_vars mutable dict to None.
Changed default for map_vars mutable dict to None.

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