Added __init__.py to make structured_grid a subpackage.
Added __init__.py to make structured_grid a subpackage.

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