Flip Flop Index Tutorial (#135)

* feat: new flip flop tutorial notebook
* Updated markdown
* Improved citation syntax somewhat with newlines and italics to demarcate things
* Review of MAE tutorial (#116)
* First review and extension

Co-authored-by: Tennessee Leeuwenburg <[email protected]>

* Update contributing.md

Signed-off-by: tennlee <[email protected]>

* feat: add proportion exceeding wrapper for flip flop index (#120)

* feat: add proportion exceeding wrapper for flip flop index
* minor updates to flipflop proportion exceeding

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Co-authored-by: Nicholas Loveday <[email protected]>

* MSE tutorial reviewed by Deryn.

* RMSE tutorial reviewed by Deryn

* Tidy of MAE, MSE and RMSE tutorials (#136)

* Split first cell into two
* Added a semi-colon to a markdown cell
* Removed one unused variable

* Added a plot to flip-flop tutorial

* Updated graph labels

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Signed-off-by: tennlee <[email protected]>
Co-authored-by: aidanjgriffiths <[email protected]>
Co-authored-by: Tennessee Leeuwenburg <[email protected]>
Co-authored-by: tennlee <[email protected]>
Co-authored-by: Aidan Griffiths <[email protected]>
Co-authored-by: Nicholas Loveday <[email protected]>