xgcm python package for working with outputs on Arakawa B, C, D grids

[navidcy]

This is related to #22:

The xgcm python package makes working with outputs that live on a staggered grids very easy! It will be a good idea to have a recipe that showcases how to do this.

Examples or discussions that we can draw inspiration from are:

• https://cosima-recipes.readthedocs.io/en/latest/Recipes/Relative_Vorticity.html#Method-3:-Using-xgcm-to-replicate-MOM5's-calculation-for-vorticity_z
• https://earth-env-data-science.github.io/lectures/working_with_gcm_data.html
• A Documented Example on how to use xgcm with cosima output is wanted COSIMA/cosima-recipes#164

cc @andrewbrown31