fix regridding bug for domains with missing lat/lon data (#162)

This fixes a bug in the visualization notebook where regridding would fail if there were no observations at a particular lat/lon coordinate at .1 degree intervals, causing uneven spacing of the xarray coordinates and failure to regrid via xesmf.

This creates a dummy dataframe with even spacing, merges it to the actual data with a left join and then generates the xarray dataset.