refactor .loc indexing for slight speedup
refactor .loc indexing for slight speedup

jhamman/xarray,hetland/xray,pydata/xarray,jhamman/xarray,pydata/xarray,pydata/xarray,drewokane/xray,markelg/xray,clarkfitzg/xray,cpaulik/xray,xray/xray,rabernat/xray,NicWayand/xray,kjordahl/xray,markelg/xray,hetland/xray,shoyer/xray,jhamman/xray,jcmgray/xarray,jhamman/xarray,shoyer/xarray,clarkfitzg/xray,petercable/xray,chunweiyuan/xarray,shoyer/xarray,markelg/xray,rabernat/xray,kjordahl/xray,cpaulik/xray