[CLIMATE-847] Support multiple observation / reference datasets in run_RCMES.py

[agoodm]

This PR will generalize RCMES to handle all datasets in the same way. Now rather than making any distinction from observation and model in the code, all datasets are loaded at once with the first dataset listed being designated as the reference for binary metrics. Theoretically, this capability was already possible but these changes should make it feel a bit more streamlined.

Todos

• Update configuration files in examples
• Test an example configuration file with only observation / reanalysis datasets.

[OCWJenkins]

Can one of the admins verify this patch?

[agoodm]

Alright, that should do it! @huikyole , please review and try out some tests with multiple observation datasets. I have just tested a few CORDEX-Africa examples and found them working just fine 👍 .