Updating the Concentration ROC diagram and adding the ROC and Molchgan plots using the alarm-based approach

[Biondini]

I added two new plots that allow to analyse probabilistic models using the alarm-based approach.
In particular i added the ROC and Molchan diagrams derived from the contingency table.
The new functions are called "plot_ROC_diagram()" and "plot_Molchan_diagram()".

I changed the name of the already availabe "plot_ROC()" function into "plot_concentration_ROC()".
The new 2 functions and the modiefied plot_concentration_ROC() are also inserted in the tutorial for the evaluation of gridded forecasts.

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Fixes issue #(please fill in or delete if not needed).

Type of change:

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• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• This change requires a documentation update

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• I have performed a self-review of my own code
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• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pabloitu]

the PR looks good to go