Source and documentation for E-P diagnostic

## The problem
As part of the Model Benchmarking Task Team, I was asked to briefly document some of the diagnostics included in the first REF release. One of them is the "E-P" diagnostic, labeled with ID 3.4 on the manuscript under public review. According to the table, this is part of the ILAMB suite, and relies upon the GPCP observational dataset. However, I could not find trace of this diagnostic on the ILAMB documentation, nor I could find the source code on the REF GitHub repo.

Has this diagnostic been implemented in the REF release. Otherwise, shall we fill in this gap somehow, and would it be possible to point towards the appropriate reference within ILAMB?

Thank you very much!

## Definition of "done"
Reference to the appropriate ILAMB diagnostic.