Created neutral density example notebook (#417)

* Created neutral density example

* Moved Neutral_density_calculation.ipynb to Examples

* Added mom6

* Fixed reviewer's comments, changed to intake and removed instructions to install pygamma since it will be incorporated to conda environments

* Have commented out the mom6 part because it is not indexed yet in intake

* changed pygamma to pygamma_n

* mom5 with intake and mom6 with cookbook

* some code cleanup

* rename

* some tweaks

* parallelised

* Delete Examples/Neutral_density_calculation.ipynb

* remove the comment about future potential parallelisation

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