tutorial notebook and script

[jetesdal]

I added a comprehensive tutorial that can be used as part of the documentation. I also prepared a simple notebook which we can used to create a script that will output a figure comparing surface WMT estimates between a model and the provided reanalysis datasets. Please review the notebooks and let me know what you think. Thank you!

[jetesdal]

Task list based on issue #11 and @jkrasting list above

• [ ] Ensure every function and class has a proper docstring
• [ ] Add appropriate DOE and Princeton Acknowledgements
• [ ] Update README.md
• [ ] Add tutorials to the readthedocs site
• [ ] Add notebook(s) or script(s) on how to generated the ECCOv4 and ERA5 datasets
• [ ] Create two tarballs (1) reanalysis datasets (+ a README file and processing scripts) (2) 5-year data files for CM4 historical run
• [ ] Create .py script(s) for the MDTF and CMEC