run_validation.sh script

[rchan26]

Produce a script for producing a number of plots/assets useful for valdiation (similar to produce_op_assests.sh).

Some early ideas:

• Allow user to pass in thresholds of interest (for binary accuracy and SIE error metrics)
• Allow user to specify region of interest
• Allow user to create leadtime averaged plots (all, monthly and daily averaged, target dates)
• Allow user to choose whether or not to compare with ECMWF SEAS5 forecast

Some of this functionality is in run_forecast_plots.sh, so maybe can extend to further utilise that too