Example of map_blocks/apply_gufunc

[tomwhite]

@TomNicholas mentioned that people at SciPy asked how to apply an arbitrary function to arrays in Cubed. An example would help users get started with this. It should cover how to estimate memory (and check if it is exceeded using #110).

[TomNicholas]

It doesn't need to be complicated - even a function applied in an embarrassingly parallel manner using vectorize=True argument to xr.apply_ufunc would be enough to show users what is needed beyond the existing documentation on xr.apply_ufunc.