Added get_centroid_of_patch.
Added get_centroid_of_patch.

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