Clean up plotting utility functions

[mnlevy1981]

One aspect of #12 is that we now decode time when opening netCDF files in xarray (i.e. ds["time"] is a cftime object); this affected the x-axis in the plots in the Sanity Check notebook, since compare_fields_at_lat_lon() no longer needs xticks, xlabels, etc. Those variables and function calls have been commented out, but they can be removed.

I also need to figure out how to use the same limits for the x-axis in all the plots generated by compare_fields_at_lat_lon().