Adding precipitation distribution for general model evaluation

[LisaBock]

Precipitation Distribution

Managed to create a recipe/diagnostics for the precipitation distribution, which yields a plot as follows:

Branch: https://github.com/ESMValGroup/ESMValTool/tree/precip_hist Recipe: recipes/model_evaluation/recipe_model_evaluation_precip_hist.yml Diagnostic: diag_scripts/clouds/diffhist.py

Some caveats:

• The different preprocessor routines pr_icon? can probably be merged to one pr_icon
• The colors are hardcoded in the diagnostics. It should be possible to assign them in the recipe as we do it for the labels.
• The documentation needs to be checked.

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Templates

(https://www.pnas.org/doi/full/10.1073/pnas.1810286115)

[LisaBock]

@agrundner24 Do you also plan to move to "frequency" instead of "density" as in the reference?