Update 20thC Reanalysis example to use xarray

[mdpiper]

This example currently uses scipy.io.netcdf, which is sufficient, but the modern way to access netCDF is through xarray. The example should be updated to use xarray.

[mdpiper]

Note that I do comment on xarray in the notebook, and defend the use of scipy.io.netcdf as an example of well-written older library.