Add function and/or example for validation plots and metrics

[cchwala]

Things we could add, based on discussion today:

• scatter plot and hexbin plots plus metrics (some metrics are calculated in pycomlink)
• take into consideration different thresholds, e.g. exclude zeros or values below 0.1mm/h
• histogram plots from FPs, FNs etc

[maxmargraf]

Here are some examples of plots and metric calculation from pycomlink:

• This notebook contains a basic hexbin plot in cell 24.
• Cell 25 of the same notebook shows the results of the function calc_rain_error_performance_metrics() from pycomlink calculating several performance metrics using a lower threshold for rainfall (for both reference and observed values). There is also calc_wet_dry_performance_metrics() in pycomlink which calculates binary metrics between two datasets, without the specification of a lower rainfall threshold
• regarding a histogram from FPs and FNs shown here, I will try to provide a simple code snippet in the near future.

[maxmargraf]

@bwalraven here is a longish example of the histogram plot mentioned above, but just looking at cell 12 and the following cells first few lines should give you the basic idea (which is not very complicated). Let me know if you have questions!