Implement aggregation of CRPS_EMP_FAIR across multiple cases #215

[TatianaBurek]

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[JohnHalleyGotway]

I approve of these changes.

I see that the logic of the newly added calculate_ecnt_crps_emp_fair() function matches the logic of the existing calculate_ecnt_crps_emp() function. And they should be handled the same way.

Since many statistics are aggregated as weighted averages like this, perhaps this similar logic could be factored out into a utility function for fewer lines of code to maintain? But the logic of the code as written, looks correct to me.