Zonal Avg Notebook Update (#1403)

* o Add another example to zonal averaging, also wrap the isel logic in a try/except so that if a caller passes a dimension name that doesn’t exist then raise ValueError; test case for the same

* o Fix missed datasetpath

* o Add comment about crossection notebook and some more fixes

* Clarify NE30 zonal mean narrative

* Clean up default latitude list

* o Fix comments and text

* o Update documentation - Zonal notebook.

* o Update documentation - Zonal notebook.

* Fix broken URL

* A few small rephrasing

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Co-authored-by: Orhan Eroglu <[email protected]>
Co-authored-by: erogluorhan <[email protected]>

Author: rajeeja