New plots to analyse Nitrogen abundance (#265)

* Adding CNO plots

* Registering new C/O and N/O - O/H relations and formatting

* Minor check

* Minor fixes, clarifying captions

* Adding new plots of N/O relations to the pipeline

* Adding C/O vs O/H plots to understand behaviour of Carbon relative to the poor comparison with observations in the gas phase

* Formatting..

* Fixing captions, adding figures for total N/O relations, and improving the functions registering C/O and N/O

* Formatting..

* Update submodule

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Co-authored-by: robjmcgibbon <[email protected]>