Adding a tutorial on how to sum fields using xarray operations

[erikvansebille]

During a discussion with @MeikeBos, I realised we don't yet show how to take advantage of xarray operations on fields. So this tutorial shows how to sum two fields (on the same grid) using xarray operations. It would be a good place to point users to when they ask about how to best adapt their fields

We could also add more complex xarray operations (e.g. vertical averaging of the horizontal flow-field), but then it would become very long

[VeckoTheGecko]

We could also add more complex xarray operations (e.g. vertical averaging of the horizontal flow-field), but then it would become very long

I'm not opposed to long notebooks. I think having one larger notebook on manipulating field data before passing through would be very beneficial (more - I think - than splitting them across different notebooks)

[erikvansebille]

Good suggestion. Now done in 2801d59

[VeckoTheGecko]

Looks good