Towards model agnosticism + pint functionality in Meridional Overturning Circulation example (#324)

* true model agnostic + pint

* run all cells

* define levels+cbarticks via method

* Minor revisions and further enhancements

* rerun + minor comments

* Include Julia's review comments

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Co-authored-by: dhruvbhagtani <dhruvbhagtani@gmail.com>
Co-authored-by: akm157 <adele.morrison@anu.edu.au>
Co-authored-by: anton-seaice <anton.steketee@anu.edu.au>

Author: 4 people