Available metrics names are not clear

[seabbs]

The eval_forecasts function has a metrics argument that allows users to select which metrics to use if the default set are not required. As it currently stands it is not clear what should be passed here and it would simplify use if this documentation was updated with these details.

The list of supported metrics are in the details section of the function documentation and I assume using these function names is the expected action?

[nikosbosse]

addressed in #203