Simplify Temperature_Salinity_Diagram.ipynb: remove model agnostic code and apply only to MOM6 (#619)

* simplify recipe: remove model agnostic code and apply onto to MOM6

* minor update to importing libraries

* added subheadings and addressed review comments

* fix cell volume calculation

* simplify

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Co-authored-by: Navid C. Constantinou <navidcy@users.noreply.github.com>

Author: hrsdawson